Firstly, to answer your question... Yes, it can be dont in 28 days.. I've dont it in 28 days, my friend has done it in 28 days, and my father would have done it in 28 days had the aircraft not gone U/S during his exam!
PAPI raises some good points about the differences, but thats not a reason (IMHP) not to learn in America... Just be prepared to spend a few hours with an instructor on your return to the UK. I think my friend and I both had 5 hours back here to get used to some of the differences.
All I would say is this... there is a lot to be said for learning here in the UK, but I think there is a HUGE advantage in the continuity that you get from flying at some of the foreign schools. Learning abroad will teach you to fly in a very short space of time... Coming back to the UK and finding a good club will tidy it all up and make you confident to fly in the UK on your own.
Go do it... You'll not look back!