Cars used to be sold with a sticker on the speedometer/Odometer saying ‘Unless the mileage shown can be proven to be correct, it is considered incorrect’. I suppose that’s how dealers got around it to cover their arse back then.
Some Mileage Blockers block the signal to the digital odometer, some sophisticated ones can allegedly block the signal to the ECU (or reduce the miles being recorded), but there are several more modules that they probably wouldn’t bother to access, such as the transmission (TCM) powertrain (PCM) and Body (BCM) control modules which have access to other distance measurement systems. They should obviously all stack up but not that many people go to buy a car armed with sophisticated code readers.
I’m guessing blockers are popular with people on a set mileage lease, or Taxi’s etc. but it’s a dirty trick all the same.
The law :
“Apparently, it’s not illegal to alter a car’s mileage in the UK - only to sell it without declaring that it’s been changed”.
What other purpose would there be for altering the mileage, other than to benefit financially.

The VIN used to be hidden under panels, under the bonnet etc., but now it’s conveniently on the dash for all to see. There should be another VIN somewhere that matches, whether it’s a sticker on a door post or a hidden one under a flap on the inner sill cover. This is all stuff that a reputable trader should check.