Dont know in UK, but here there are some companies that remove the milage, but they are pricey and there main job is to “align“ the clocks so they dont lye.
Just done this with a Vectra. What I did was put the original mileage on a sticker and the mileage of this one when it was installed. Stuck it on the bulkhead. The new one was 8,000 less than the one taken out