If it was me I would try my SIM in another phone to see if the handset was faulty
See if anything in these guides helps
Check for network issues here O2 Service Status
Trading Standards will not get you anywhere. The T&Cs of your contract with O2 do not guarantee you any service at all ; and that is the same across all of the UK mobile network operators.
If you feel strongly, you could complain to O2 :-
In view of the difficulties that you have faced, you either want them to add an ex-gratia credit to your account, or to release you from your contract early. I have no idea what the outcome will be, but you might consider it to be worth a try.
If you intend moving to another network, I suggest that you purchase some PAYG SIMS and test the other networks in the areas that you frequent. Otherwise, you might move to a network that is no better than you have got at the moment.