Yes, it�s perfectly legal. In fact it was not that long ago that this was about the only way you could be detected speeding.
To secure a conviction the prosecution has to produce evidence which is sufficient to convince the magistrates beyond reasonable doubt that you were exceeding the speed limit. The testimony of a police officer who checked his speed whilst following you at a constant distance would normally suffice, especially where the excess speed was considerable as in your case.
You might be able to argue that the distance between you varied and so his speed must have varied from yours. It is unlikely that a Bench would accept that the accuracy could be 60% out, which it would have to have been to bring you down to 60mph. You might also question the calibration of his speedometer. Police vehicles usually have sophisticated, regularly calibrated equipment in them to lessen the likelihood of a challenge of this nature succeeding. Again, you would have to show the estimation to be around 60% adrift.
There have even been speeding convictions (one quite recently) where motorists have been found guilty based upon the evidence of a person standing on the pavement. So long as it can be shown that this person has the experience and ability to judge a vehicle�s speed sufficiently to say that it was exceeding the speed limit, the prosecution can succeed.
Exactly what constitutes sufficient evidence is for the CPS to consider before bringing the prosecution and for the Court to consider when presented with it. However, contrary to popular belief, it is not always necessary to show by precisely how much the limit was exceeded, nor is it always necessary to use sophisticated electronic devices.
Jake, as usual, has got the sentencing guidelines spot on, so you can look forward to at least 6 points.