Wagermall isn't great. I have read about numerous CS problems. I was a player there for about 1 month, lost my balance and wont go back. Also, I think a lot of their books are NASA related.
Stan James are a decent UK company - i use them sometimes for UK Jumps racing - early prices are sometimes opinionated. No problems paying etc but may restrict larger successful bettors but thats to be expected. Call centre staff are knowledgable too.