The sooner you make the appointment, the sooner you'll get seen. You can go to NHS walk in clinics, your GP, etc. The best people to initially call to find out where individually, you'd be best off going (depending on how comfortable you feel with each of the options), ring NHS direct, whose number is in one of the sticky threads at the top. Explain to them that you need to see someone to ascertain whether or not you are pregnant and you're quite anxious to get seen quickly, and they'll list your options for you in the area you live.
Then book an appointment if necessary, or just find out times etc and go along to the clinic. I'm masterful at putting these things off, but seriously: just ring them now, and get it booked. It'll be a weight off your mind.
Gem xx