I am pretty sure that mileage is not changed when its imported.
For example, when i purchased my JDM it had 39,000 on the clock, for me to work out exactly how many miles i just checked the mileage in the service history when it was imported.
I would say the Speedo is as accurate as any UK car, it will always be slightly out, to get a true reading of this you would need to use GPS to determine your speed.