Its a pain in the ar$e.
You will need to make it fully road legal to the nth degree, tyres, lights, indicators mirrors ect.
You will then need to book it in for a vosa test to be inspected.
IF it passes their testing/inspection you will need to get it insured and MOTed on the frame number.
Then the DVLA will want you to take it to your local DVLA office for them to inspect (in my case just to measure the seat and sump height) before they will issue you a numberplate (more expence) you have to tax it on the frame number and they will transfer the tax to the new plate.
I have done this in the past with crossers and unregistered enduros and its too much of a pain to bother with.
The best bet is to buy a road registered crosser or enduro.
Dealing with road registering offroad bikes is a long painstaking and expensive task
