Hi Ian:
Based on what you have said above, I recommend that you consider the following issues as you go shopping for a GPSR:
1) Get one that has all the cartography for Europe preloaded into the unit, for example, a 2610 with a big data chip, or 2720. There are also some newer, smaller units that come with all of Europe preloaded - I'm not ruling them out by not mentioning them, I'm just not familiar enough with them to recommend them. There are many other members here who are right on top of developments with the smaller units - I am sure they will add comments. Having all the continent preloaded will save you untold grief worrying about what map segments are on your data chip, and you won't have to lug a laptop around to load the GPSR as you move around. Based on my own touring experience (across Europe every summer, sea to sea), I think the advent of GPSRs that have the whole continent preloaded into them (or can hold the whole continent on a single data chip) has been the most beneficial advance in technology in the last 5 years for touring riders.
2) Get one with a reasonably large screen. If possible, try riding with the GPSR for a day to see how comfortable it is for you to view it and control it. Some of the newer units are very small - this is not a problem if you have the GPSR installed in a car, and you can move it around as you wish, but the GPSRs with a really small screen can be frustrating and difficult to use on a moto, when they are mounted in a fixed position, and you are trying to read the display through a rain-soaked visor. I'm kind of partial to the 26xx and 27xx form factor, but maybe that's just because I'm used to it.
3) I think you probably should confine your product search to the 'pure automotive' GPSRs - these are the ones that generally don't come with batteries in them. Those products are listed here:
Garmin / Mobile Electronics / Automotive.
4) Although it sounds almost silly to mention it, be sure to get a GPSR that is waterproof. There are some really innovative, neat new products on the market now, but not all of them are waterproof.
There is, unfortunately, no easy answer for the needs of users who want 'two continents' coverage. Back in 2002, I purchased an initial unlock of CityNavigator Europe for my Americas spec SP III at a cost of about USD $600. That really didn't give me the full capability of an Atlantic SP III, though, and the situation hasn't changed much since then. For that reason, I suggest you not try to get a GPSR you can use on both sides of the Atlantic. Instead, concentrate on getting what is best to meet your needs in Europe, and when you do make those occasional trips to America, either borrow a GPSR from an American owner who has the same one (i.e. you pair up and swap use of your GPSRs when you are in each other's continent), or, just buy a current Americas model new, and sell it on eBay when you are done. Although that sounds extravagant, it will probably be cheaper than buying cartography for both continents.
If I had to make a specific recommendation based on all you have written about your planned use, I would suggest either a 2610 with a 2 gig CF card, or a 2720. The user interface is very different between these two series - almost like the difference between DOS and Windows. Be sure to try both, and try to determine if you have a strong preference for one UI over the other. I don't think that many of the new features that have recently been announced (RDS traffic information, speed camera databases, etc.) would offer much benefit to you - we can split lanes on motos, and it's kind of pointless to try and maintain a database of all the speed cameras in Europe. But, if you plan to use the same GPSR in a car and drive in the urban areas of the UK, those features might have some value to you. They are available with the 27xx series but not the 26xx series.
Michael