The kit lense with the 40D is 17-85, that's quite a good range and costs about £832
You don't have much left in the budget after that, if you like fast lenses maybe a prime would be an option?
Remember that lenses will outlast several camera bodies so at your budget I think your money would go further if you went for the 400D/450D and spent more on lenses?
For low light is that indoor or outdoor? if indoor consider looking at a t 430EX, this can really help pics indoor with bounce flash