What to see in London

Hello, I'll be in London in two weeks for a couple of days and I'm curious, whether there are some interesting ehibitions/shows either temporary or permanent, that would be worth visiting. I am mostly interested in "classical" craft - large format/analog work/alt processes....
Thank you!

I used to read BJP every week and it had a great round up of the current exhibitions in London (and other areas.)
And my favourite magazine http://www.professionalphotographer.co.uk/ has listings also.

http://www.professionalphotographer.co.uk/Exhibitions

In the past, the National Portrait Gallery has had some wonderful exhibitions, and there are a few small galleries, but their names escape me??