If I build hours in the US, lets say 20 hours in two weeks
At your stage you will not be hour building but learning how to fly. The style of instruction is quite different in the USA to the UK so interfacing into the UK system on return will not be seamless. You will need 25 hours dual instruction with a JAA FI.
The UK CAA will not be issuing JAA licences after 30 June so you will be looking at an EASA licence however; that won't make any significant differences.
For two weeks flying, the paperwork is not likely to be worth it.