There are four main problems which you will encounter. The first is the R/T differences, buy yourself a cheap airband radio (about 18-20 quid) then sit on the end of the local controlled airfield and listen to how the local folks do it. This will save you money since learning UK R/T will cost you loads if you learn whilst flying. The second difference is the peculiarities of the UK overhead join into the traffic pattern, (I am amazed that this is considered to be a safe manoeuver!) again it's cheaper to learn this on the ground with a pencil and piece of paper in your hands rather than a yoke. Then there is the language differences, patern = circuit and then there's this QFE thing! The third problem is the prejudices which UK instructors have about people who have trained in the USA and the fourth is coming to terms with the amazing high cost of flying in the UK!
If you have trained at a non towered airfield then I would definitely recommend spending several hours at least with an airband radio at a towered airfield familiarising yourself with the R/T.
And CAP 509's suggestion will save you money too.