Yep - sign up, then whenever you buy something online, go to the site through quidco and you'll earn your cashback.
As far as photographic stores there's Jessops, Warehouse Express, Jacobs plus some other regular electrical stores.
Be aware though, when pricing stuff up it's too easy to go through Quidco but you might still be able to get things cheaper.
Oh, and if you try and use discount codes, you won't get your cashback - there will be a list of any codes you can use with quidco before you go to the site. For example, there's "quidcofive" for Jessops which gives you 5% off everything except lenses and DSLR's
There's also a couple of stores where you can get cashback by registering your credit cards with Quidco then when you buy from the store, you get the cashback - Halfords is one example. I've used it and it works.