The problem with a "better deal" is that something that is good for me might not be good for you. As with most mobile "good deals", you should be looking for lower payments and ideally more minutes/texts AND less money!
I'd start by calling O2 and saying that you're thinking of leaving for another network, saying that it's costing too much and see what they offer you. From your current usage, that works out to around �15/month ... if they offer you something for less than that that includes enough minutes and texts so you don't have to pay for extra, then that's the better deal for you!