I assume you drive on the left in the UK. In the US we drive on the right and
on "Flat Roads" cars pull to the right. I think it is because even flat roads are not level so that rain runs off. If you had to steer right to overcome the pull
then your alignment is in question. Did you have a 4 wheel alignment?
Or unequal tire pressure can cause this. You could always try a trip to
a country where you had to drive on the right and see if it pulls to the right.

The comment "They all pull left" is only half of the correct answer.
Have you driven on the same road in a different car with your hands off
test???

Hope this helps