Of the 3 main comic stores in London, Orbital would be my pick for the 'best' one. Decent back issue section and some OA as well. They also have an exhibition space which occasionally has some interesting stuff. Probably the best all-round comic store in the capital. It's long been my place to pick up the latest comics.
Gosh aren't so great for back issues but they do have some really fun, quirky stuff that you'd be hard pressed to find anywhere else. They also have some nice OA available too.
Forbidden Planet is as corporate as they come and, as said above, should be avoided.
30th Century Comics is further out from the centre but not too far out. Tons of back issues but quite a lot of pence copies and grading is not brilliant. Good place to pick up non-keys in average grades though.
US based comic collectors (and indeed any collectors from outside the UK) are probably best advised to come over here when the London Super Comic Con is on. It's the only con we have which comes closest to what you have in the States. Dealers such as Bob Storms and Harley Yee are regulars as are Metropolis. Usually a good range of guests too although it hasn't been too strong on Silver and Bronze Age creators in the past couple of years IMO. Plenty of accommodation nearby and although the surrounding area isn't great, you can get into the centre of town easily and fairly quickly.