I had no idea they were 2 diff sites, I have played a little at William Hill but never been too impressed with them. They used to run some promo's alongside Mansion Poker
I think, all I can think to try is look around their site and look for some affilliation.
As a UK player, i'd have to suggest getting onto Ladbrokes instead. Quite a bit softer (though a similar number of players), and GREAT guaranteed tourneys.
Plus a few unusual low stakes SnG games you wont find elsewhere which are loads of fun.