I've just been re-reading some of the other answers.
Tom is in error if he thinks that there's a statutory minimum speed on motorways (except where there are signs specifically stating such a limit). There is no such limit.
However, I agree with Tom that the boys in blue will still (rightly) take an interest if you drive significantly slower than other drivers in the nearside lane.
Monster-S, quite frankly, terrifies me! I hope that I never see him in my rear view mirror!!! The absolute minimum distance you should keep between you and the vehicle in front is the 'typical stopping distance' for that speed.
That means that, at 40mph, the absolute minimum distance between vehicles should be about 9 car lengths.
At 50mph, that figure rises to 13 car lengths. At 60mph and 70mph the figures are 18 and 24 car lengths, respectively:
http://www.highwaycode.gov.uk/09.htm#105
Chris