No idea about A/C's here, but in India, where it's literally boiling
I ahve some experience
First off, depends on how much you wanna spend. You have a choice between a window AC and a split. The window, funnily enough, goes into your window and makes more noise than the split, but arent overly so. I have plenty of Hitachi's at the house India,s o can recommend them. Pretty silent and good.
The split's go on the wall, with a small pipe going to another uni, usually kept outside the house. You may have to make a hole in the wall depending on the location. Very silent, you won't even hear them. The hitachi one I have is good. Daikin are very good as well. They are more expensive than window though. But for a large room, probably better since they can be more powerful.
No idea where to get one from though, so you'l have to look around that yourself
Oh also, sometiems, the palces where I lived, the temperatures went up to 48 degrees. Unlikely to happen in this coutnry, so you won't need something as powerful. That should bring the price down a fair bit. On the other hand, London is just plain expensive so taht might just offset any price advantage