That's a question I desperately want answered myself. Last year I was in Tokyo, where there is NO fucking metal scene as far as I'm concerned, yet still there was an awesome shop that I found which had exclusive, mail-order-only type releases all over the fucking shelves. It made me realise that in 20-odd years in London I've seen nothing like that...SHAMEFUL!
The shop in Camden you're talking about is Resurrection Records, I believe. They're really not very good, with little you couldn't have gotten from from HMV at some point.
If anyone knows any good places, cough em up.