Hi Nate, I am in Berkshire (20 miles from London west side) - CRF450 rider..
"the grubb" explained quite well what you have to do to register your bike here in UK..is not that easy... takes time.
I spent 6 years in Texas and I was riding in the Houston area. When last year I moved back to UK I brought with me my dirt bike and my road bike. So I went through the whole iter than grubby described above for the road bike. For the dirt bike, unless you want to use it on the road, you have nothing to do. Only MX track anyway, no enduro. For enduro the bike needs to be at least registered with the DVLA.
Be aware that riding in the US is a lot different than in the UK... It was much better in TX. I use to ride to several tracks always prepped, minimum cost, easy to reach from Houston. Here is always a struggle to do any off-road. Right now looks like there will be for 2006 even more restriction on the use of UK countryside.
Also the tracks are quite in bad condition most of the time and at least 40$ for practice.. if not 50$...
Still, if you love dirt biking..you can not stop... like me..