G'day Scutfarcus,
So you're moving to the UK, "isn't that lovely".
I'm not sure if the BA program in the UK gives you any mileage credit for discounted airfares. You may want to check that out first. I believe the USA program offers more, including the household account. It may be better to keep to the American program and if and when you have accrued enough miles, transfer to the UK program to claim an award.