I'm NOT an accountant or tax specialist - hell, I only just about stagger from pay check to pay check, but what I do know is that we have bought and sold three houses here in the UK and all the profit was totally tax free - hooray! There was no need to declare it on my UK tax returns.
There is something here called 'stamp duty', which is about 3%(?) of the purchase price and is paid to 'register' the new owner of the property, so it's a tax in all but name. But this would not apply to a sale you had made outside the UK.
I can't see how the UK would tax you on any money you made in the US from the sale of property, but like I said, I'm no expert, if you're still concerned, I'd strongly recommend you see a UK based independent financial adviser - where are you moving to??? Someone here may be able to recommend one?