If a motor trader sells a car that's had the mileage reduced, it
can be treated as applying a false trade description, pursuant to the Trade Descriptions Act 1968. There was a test case back in the mid 1980s where a car dealer had clocked the cars himself, and then put a disclaimer sticker over the mileage, which said the accuracy of the mileage recorded cannot be guaranteed. On appeal it was held that because he had clocked the cars himself, he could not rely on the disclaimer, and he was guilty of the offences. He got locked up for it as well if I remember rightly.
http://www.wigan.gov.uk/Services/Bus...raderguide.htm
For private individuals it's somewhat different. If a private individual was to alter the mileage of their own car, either by winding back the existing speedo, or fitting another one showing a lower mileage, then it depends on what happens next. If you did it and sold it on, stating the mileage was genuine, then it could be said that the mileage was one of the terms of the contract of sale. If the buyer found out, you could be sued for breach of contract. Or prosecuted for fraud.
If I'm selling a car, it depends on what I know about it. For example, that yellow fiesta I've just finished restoring has 45,000 miles on the clock. It's one of those clocks that only goes up to 99,999 before going back to 00,000. I've had mk2 fiestas in the past that were on their second time round. But the yellow one has service history which suggests the mileage is genuine, plus I've looked on the MOT website and the mileage checks out on there too.
I've had cars over the years that I had no way of knowing whether the mileage was genuine or not. So I've just told potential buyers exactly that. I mean, unless you've had the car from new, how do you really know if the mileage is genuine or not. I had a jag a couple of years ago with quite a low mileage on the clock. In the service history folder there was a letter from a car dealer in London stating that it had had a clock change at a certain mileage, (due to digital dash fault on early XJ40s). I just added the original mileage to what was on the current clock, and explained this when I advertised the car.
For private sales, to a large extent the phrase Caveat Emptor (let the buyer beware) does still apply, except if you're selling a car and don't want grief, don't take the piss and tell blatant lies. If the speedo cable snapped and you didn't replace it for a couple of months, no big deal in the scheme of things. But if you fiddled the speedo and 'lost' 50,000 miles between MOTs, and didn't tell the buyer or potential buyer, you'd be on thin ice if they found out.