The list of ‘Must have’ things is very short:
Suitable Travel insurance. Only a real mug would go without this.
Reciprocal Health Care Card
https://www.nhs.uk/using-the-nhs/he...-a-free-uk-global-health-insurance-card-ghic/ Again, only a mug would go without this.
Valid passport, check the dates:
Valid driving licence
V5
Certificate of Motor insurance
Breakdown / recovery insurance. Again, only a mug would go without this. See other comments in this thread.
Any prescribed pills and potions and your glasses. All as applicable. If you don’t have prescribed pills and / or don’t wear glasses, delete this item,
A keen awareness that they drive on the right.
That concludes the ‘Must have’ things.
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The ‘Nice to have’ list can be as long or as short as you like. For example there is no point in taking any tools or spares, if you have no idea how to use them. Likewise, having a puncture repair kit / compressor is equally useless, if you have no idea how they work. So let’s concentrate on the useful things.
Money, preferably in euro denominations and some Swiss Francs, if you go to Switzerland. You won’t need hundreds and hundreds of pounds worth. Why? Most places will be quite happy accepting a card payment. Just have enough small denomination notes / coins each day to buy a coffee and a bun.
A credit card. A cash point card. Don’t spend hours agonising over “Will my bank charge me a fortune?” You very probably won’t be spending a fortune. That said, there is a sticky here on UKGSer on what is ‘best’ at any one moment.
Some idea of where you are going. So, either a map and / or some sort of navigation device and, preferably, half an idea of how to read / use them.
Waterproofs. Nobody knows what the weather will definitely be like. If you have got them and never use them, great. In short, expect the best, prepare for the worse. Any idiot can be cold and wet. The same can be applied to keeping your passport, V5, certificate of insurance dry, too.
Mobile phone. Mains charger (with Continental adaptor) and maybe a modest charging brick thing. Why the latter? If you have a problem, miles from anywhere, there is little more annoying than your phone’s battery dwindling away.
Motorway blipper thing.
You’ll end up taking too much ‘stuff’, that’s for certain. The two Golden Rules:
1. Pack once. Unpack. Chuck half of it away. Repack. Unpack. Chuck half of it away. Repack….Repeat as necessary.
2. Halve your kit (see point 1) and double your money.
If nothing else, you are going to be entirely in Western Europe. The place is pretty much the same as it is here. They have shops, restaurants, police stations, banks, ATM machines, petrol stations, hotels, doctors, dentists, taxis, airports and hospitals, too. If you find you have forgotten something, they will sell it. If you need a doctor, they’ll have one. If you have a real emergency…. Your Travel / Breakdown insurance will sort it out and help you. Don’t go without it.