HI Aebous, just found this thread thanks to Mike. As you know having been in Germany, the German SOFA doesn't require US personal to modify their cars to meet EU regs. In the UK, US personal need to not only have their cars conform to UK regs, but also they receive a UK title (known here as the V5). You still keep your original State title allowing re-importation into the US. Road tax for our 2003 Z4 is 185 pounds a year, I think that being an import into the UK is why it may be priced different than UK Z4s.
Here's what my Zed needed for the MOT (MOT= annual safety inspection for cars 3 years old or older):
Rear foglight (I wrote a 'how to' but I have not yet submitted it as a post, still working on it).
Front clear marker lights (AKA Parking lights).
Headlight: During the first MOT the garage aimed the lights low and passed me while during the second MOT, the garage told me the 'kick-up' of light toward the right was too high and the lights were aimed too low. After fitting some stick on 'light deflectors' (found at Halford's, a store similar to PepBoys in the US) they eventually, and somewhat reluctantly, passed it. I'm currently looking to buy UK Z4 headlight projector inserts to replace the US version (my headlights are non-Xenons FWIW).
Before moving to the UK I did some research on the MOT requirements (view these two sites; http://www.ukmot.com/index.asp http://www.motuk.co.uk/manual_910.htm) I then done the changes in the US before I arrived in the UK. I was able to do them without interfering with the CAN-BUS or car's electrical settings, didn't need to visit the BMW dealer for any reprogramming and the rear fog light looks/is OEM without any obvious add-ons (except the LED front marker lights) all the while costing me a measly $50 or so. It took a weekend of work (after a couple of weeks of deciding how to add the rear fog light), but if you're handy with a wrench and patient you can do it no prob. Alternatively one could go to a UK BMW garage, where they would reprogram your Zed, add needed wiring, headlight switch, new headlights etc. Obviously this would be quite expensive not the mention of undoing this in a few years when returning to the US.
Insurance: I would go with USAA or Geico and not a UK firm. Nothing against UK firms it's just that we do not have a full UK driving license and I have not yet found a UK firm that will insure a driver without a full UK license. As US personal under the SOFA in the UK we get a supplement to our Stateside license (make sure your Stateside license doesn't expire before you leave the UK).
We'll keep in touch,
Mark