My OB always went by my lmp to date my pregnancy no matter what the fetus measured...even when I measured 2 weeks ahead (by growth scans) from 20 weeks on.
My OB's ultrasound machine and the us machine at the high risk OB I saw, both have a +-6 beside the date when they measure the fetus. They told me it's bc the date can be off 6 days either way and that's normal. they may do things differently in the UK, tho.