Fish in London?
We had lunch the other day at the Blueprint Cafe (right on the Thames.. great views) and there was a lot of fish on the menu. My husband had a grilled, whole sea bream and thought it was nicely prepared. From what I remember, they also offered grilled mackeral plus 2 or so other things.
Depends really on your budget but J Sheekey, Scotts and Bentleys will be your best bet. There's also Sweetings, which I haven't been to personally but crops up regularly for fish recommendations.
Most good restaurants will of course offer fish on their menu, but the ones listed above are known for their excellent quality of produce and cooking.