There should be shops in the UK that test and calibrate speedometers and I'd think it would be a minimum cost to have one test the speedometer to see if it is functioning correctly or not functioning correctly.
If the problem is in the drive gears in the trans check this site for info on how to calibrate the drive gears
http://www.transmissioncenter.net/sp...n_______va.htm You will have to find the right gears for your transmission on the list as the page covers a big number of vehicles and brands.
You may be able to source the correct gears locally or get a ratio adapter locally that has the correct gears in it to correct your speedometer. Those are often used here by the four wheel drive people who are always swapping tire sizes.