Iceland used to do quite a bit, but I was disappointed on a recent trip to my local when I found the choice more limited than usual.
Have found my local Poundland to be very good - do crisps in various flavours, crackers, cakes, biscuits, bagels (but steer clear of the cup cake mix if you go in there). Can be hit and miss - they'll have one thing one week but not the next, but I never come out empty handed!