I know this is an old post, but the info here might still be relevant.
A few years ago I had a KX500 on the road in Scotland.
I did go over the top as far as making it like a normal road bike ie: indicators alloy wheels, idiot lights and a colour television on the instument console.
Go to www.craighood.co.uk
for loads of info, magazine articles, photos of the bike.
I don't know of any changes in the law up to now, but around 1994 it was very easy to make a motocrosser 100% street legal.
All you really needed were trail tyres, nothing else.
You didn't even need lights, a speedo or a reflector, as long as the bike was driven during daylight hours.
The noise level of the exhuast might come into it depending on the mot station.
I also went over the top trying to quieten down the exhuast.
To get mine on the road (before I customised it), I had to :
Take it to the main police station to get the frame and engine numbers checked for theft.
Get it mot'd, road tyres and a plastic guard over the front sprocket (about £5)
Then register it (I ended up with a Q plate), and insure it (had to phone around a bit to get it insured).
I would recommend it if you want a bit of fun and thrills, but my bike in particular was not very practical.
I got roughly 20 miles per gallon and the magneto coils kept breaking sometimes leaving me stuck.
Rebuilding the heads regularly is only done when using the bike in competition as you want the engine in a high state of tune.
I've never touched the barrel on mine.
I enjoyed every minute driving mine mostly on one wheel till I got caught wheelying at around 70mph in a 30 zone (no traffic or people about).
Banned for dangerous driving and ordered to resit every test I had passed before.
I had to resit my car test and since then I haven't resat my bike test as it costs a fortune.