Currently I have a build being done by pc specialist and the processor I have ordered is out of stock. They don't know when it will be available and I can either wait or change to another. I admit I'm not 100% clued up on this but this is what I have in order
The 3550 is slightly faster ( 3.2ghz v 3.3ghz ) probably only a couple of pounds difference better than waiting for you maybe? The 3570K (3550K?) is similar but can be overclocked.
The 3770 and 3770K (overclockable) are both i7's and cost a fair bit more money than the i5's so it will bump the price up maybe by even £100 I would expect, they are quad core WITH hyperthreading and the i5's are quad WITHOUT Hyperthreading, that is the difference. Good Luck anyway