Drive at 100mph+ Legally How?

As far as we can work out, only three ways exist to use a car/motor cycle at speed in the UK.

Become a Police driver. The Police are always looking for new recruits, but if this is your only reason for joining then you may be rejected!
Take part in a Track Day.
Go to the Isle of Man. On the roads outside of towns where you see a derestricted sign it means just that – NO speed limit exists !!
Track Days. Use your own vehicle or one rented from the Track Days organiser and drive at one of many circuits in the UK and abroad. See TrackDay Listing for details of all scheduled car and motor cycle track days.

If you ride a motor cycle then you must read Twist of the Wrist II, a superb book that will almost certainly make you a faster, safer rider on road and track.