DontSpendIt.net

The key to these is the average income per hour you would expect to receive.
e.g. £100 bonus that takes 20 hours to qualify for = £5/hr,
£10 bonus that takes 10 minutes registering on a site = £10/hr (as max you can receive is £10)

Some sites require large (20x or more) play-through requirements before you can withdraw, so always research first.