As said; it depends on what kit your mates have. If it's radio then it's likely to be PMR which is essentially uses the eight licence free UHF frequencies which can be used in Europe.
If it is PMR (sometimes referred to as PMR446 or just 446) then you have a number of choices open to you. Whatever you do don't be tempted by the cheap Argos/Maplin style PMR radios and the so called motorcycle headsets that Maplin and eBay traders sell.
The radios mentioned above might be OK for family use and for the kids to play with in the garden but you'll have problems connecting them to a decent intercom or headset. Been there, done that, waste of money...
The cheaper (i.e. less than £50 new) motorcycle radio headsets most likely don't have noise cancelling microphones. Having been on the receiving end of such a set-up; wind noise is obvious above 20mph and conversation not possible above 40-50mph. The user of the headset might well be able to hear everyone else OK but everyone else will ask you to chuck it in the bin at the first stop.
PMR radios are generally compatible with each other across manufacturers though the scale for setting for the privacy tones
CTCSS may differ from one to the other. You'd only need to find that, for example Kenwood channel #3 is 446.03125 MHz (PMR ch #3) and 103.5 Hz for the privacy tone.
Don't be fooled that the privacy tones a/ give you privacy, they don't. Or b/ create sub channels on the 8 analogue PMR channels, they don't. They do help to eliminate cross-talk in crowded areas of radio traffic but raise other issues.
Buying PMR radios:
Kenwood TK3201, or it's successor, is the de-facto standard. Though military spec. radio. 16 pre-set PMR channel/CTCSS tone combinations. Specialist knowledge required to re-tune but usually riders with other kit use one of the 16 channels anyway. Reputably Kenwood ch#3 is the unofficial bikers channel... About £180 new, £60-£70 on eBay.
Other makes such as Motorola and Incom sell cheaper than Kenwood but comparable quality. Usually the same restrictions on retuning the PMR frequencies though the ability to manually set the CTCSS tones maybe there.
The above radios tend to adhere to the OFCOM legal requirements for PMR use. Namely; ERP of 0.5 Watt or less (ERP is power output factoring in antenna gain), fixed (i.e. non user changeable) antenna, can only operate on PMR frequencies.
The first two of the above limit range and the fixed antenna does compromise positioning of the radio on the bike. Putting the radio under the seat for security, laying it horizontally, will drop range by a considerable factor. Eg; Upright - 1 mile, laid over - a couple of hundred yards. It is legal to adapt a legal PMR radio to use an external antenna so long as the ERP remains at 0.5 Watt or less though you'd need the knowledge and willingness disassemble and modify your radio. I have known people who have done this to Kenwood TK3201s.
The alternative is to take the legal risk and buy a radio from Hong Kong/China that flouts the PMR regs. These are user programmable (within and outside PMR frequency range), have up to 4 Watt ERP and have detachable antenna. Not that I countenance use of such kit but the chances of being caught are exceptionally slim, especially as you are a moving target. Would seriously dissuade business use (i.e. instructor) though as you'll probably operate in the same general area and the consequences for you business if prosecuted could be pretty damaging.
Radios such as the Puxing PX-888 fit the bill perfectly, costing about £50 delivered. It uses the same accessory socket configuration as the Kenwoods. If going this route be sure to order the UHF model, not the VHF.
One of the important aspects to consider is that of connectivity, you need to be able to connect the radio to your headset or intercom. Cheaper Argo/Maplin radios might well have proprietary connections for an earpiece that you won't be able to hook-up to the rest of your kit.
Buying headsets or intercoms:
There are exceptions to the headset rule in so much as there are some out there with proper noise cancelling microphones. Autocom made a kit specifically for use with a PMR radio (no other inputs) though I believe that this is now discontinued but they do crop up on eBay every now and again. Another is the
McKay Claw system which I have direct experience of as well as Wildtalk who were very helpful. Use of the open face system in a full face helmet is recommended as the full face kit has a less effective mic.
Of course a dedicated headset will just do radio only. If you want to connect other devices then I recommend Autocom though there are other competent systems out there. Either buy new or off eBay, aim for the Super Pro Avi as that's the most versatile without being too expensive. About £100-£130 should get you one. Look for people selling with additional leads that you might find useful. Other than the headset/s and basic headset extensions, which should be included in the kit from new, extra leads to connect other items and things like PTT switches are purchased separately and the cost does add up, even when buying used. If you can buy a used outfit with ones you require included so much the better.
Headset or intercom I would very much recommend using a PTT button. VOX, even with a decent intercom, will be prone to inadvertently transmitting and causing annoyance to others in your group. Never set the radio to use it's own VOX as that would probably result in it practically being on permanent Tx.
A decent headset or intercom should enable you to communicate at all reasonable speeds. I was getting audible comms on the German Autobahn last summer at speeds in excess of 100 mph. Sure wind noise does make it harder to hear but I could just make out what our TEC was saying. At 70 mph everything (PMR, Satnav, MP3 and phone) are all perfectly audible for both me, and in the case of PMR and phone, for the other party. Placement of the helmet speakers is vital to ensure you can hear what is being said. Follow instructions and be prepared to experiment with this until it's optimised, it makes a world of difference to audibility above 50-60mph.
Antenna:
If you want an external antenna then the
BO antenna kit is recommended (long, long thread. Read first few pages and then skip to the end). We were getting a good 5km, in France last year, via PMR using these antenna.
Costs - overview:
With a used, or Chinese, radio and a McKay headset you could be up and running for about £120. With similar radio and used intercom about £170-£200. If buying new radio and intercom then you could be spending £500 or more.
Bluetooth:
If your mates have Bluetooth equipment then you will need to buy yours from the same manufacturer to ensure compatibility. Also be aware that there will be a limited number of users in your group that can pair at the same time. Limited to 4 or 5 though this does vary between manufacturers and models. Range will be significantly less than a decent PMR set-up at around 1000-1500 metres. PMR and intercom can all be bike powered if you like, Bluetooth will need to be charged daily when on tour, not easy if camping.
Range:
Don't believe marketing hype about range for PMR radios or Bluetooth systems. These tend to be theoretical and for ideal conditions (optimum atmospherics, line of sight mountain-top to mountain-top). To avoid disappointment in the real world, with a good set-up, expect no more than a mile or two with PMR though this will vary depending on terrain, wooded areas etc.
Whereabouts in Gloucestershire are you? I'm on the Gloucestershire/Herefordshire border. Would be willing to show you my comms set-up and chat if you like.