There is no point in sueing them as, presumably, you haven't yet paid the remainder of the contract. Realistically, you should consider money that you have already paid on a monthly contract as gone. One option is to ask for a settlement figure for the contract and this should not be more than the total of the next 22 monthly payments.
For the contract to continue, you both have to keep your sides of the bargain and, if you don't, you have to be given a reasonable time to put things right. If, for example, you missed the next payment, they would give you a little time to pay before declaring you in default of the contract. Simiularly if they don't provide the advertised coverage, you need to give them a little time to put things right before you can justify stopping payments. The coverage maps of each supplier are on their web sites.
If you do stop payments, it is quite likely that the provider will get awkward so it is important that you prepare for this by putting details of the problem together with details of your calls to O2 in writing. If you were starting from scratch, I would suggest one letter detailing the problem and asking for it to be addressed. Then, after 30 days, another letter stating that you will not continue payments unless they fulfill their side of the contract by rectifying the problem, then, after another 30 days, if things didn't improve, I would stop payments. You can follow up such letters with a phone call or two but some suppliers have a habit of saying one thing one day in a phone conversation and then denying any record of the call the next day.
If you do something similar to the above, you should be prepared (at worst) for nasty red letters, nasty phone calls (if you can get connected!), black marks on your credit file and threats of court action. You should not take such action unless you are prepared for this by keeping copies of letters and posting receipts and by having the right attitude. I appreciate that some people prefer to keep paying and to not make waves in similar situations.
Another time, if you are taking out a phone contract, you should consider ordering by phone or over the internet. You could then test the product in the first couple of days and, if things were not up to scratch, you could cancel without penalty during the first 7 days under the distance selling regulations.