If you don't need texts, then i wouldn't bother with FleXt- isn't much gain unless you mix and match both.
My advice would be to buy from phones4u rather than CPW if you are going high street shopping, the guys there are on higher commission to get deals, so they are much more willing to give you extra bonuses if you haggle
If customer service is of no concern to you, then you might want to consider three-
http://www.three.co.uk/priceplans/paymonplans.omp