Alex,
You should DEFINITELY start with the William Hill bonus instead. 5 pounds per table hour, to a max of 25 pounds. So play 4 tables for an hour and fifteen minutes each and you'll get the full 25 pound bonus, or about $44. A really good deal.
Then move on to Interpoker. I kept track and their bonus clears at about 35% to 40% the rate of the WillHill bonus at $100 NL full ring. 1.25 hours of 4 tabling will get you a little under $20 most of the time. Not bad.
The $100 NL games are pretty soft. They're not Bodog- or Martins- crazy type games. Poor players all over the place, but they're weak-tight as often as they are crazy-maniac. It seems pretty common to run into short stacks who push all-in on the flop with 87o that hit second pair against your AA that you raised preflop. I don't know if they think their last 5 bucks gives them folding equity or what....