Well you can just go in to any NHS dentist which is accepting new patients and register then make an appointment, which will unlikely be for today however but still. Ask to be put on the cancellation list, so you can get offered an earlier appointment if someone cancels. Some dentists have emergency drop-in hours so check that out. Once I was in so much pain, I just went to my dentist, explained the situation and they agreed to see me once they'd seen all the patients for the day. Mind you, I was already registered, but its worth a go.
In the meantime, take painkillers to ease the pain. Go to the pharmacy and try and get some behind the counter stuff like Paramol which should be more effective than just paracetamol, aspirin, ibuprofen or whatever it is that you're already asking.