To be honest, I'd go to Tesco's or Boots. Tesco's do 2 for £4.70. (Got some yesterday ) and they are pretty good in my experience. Apparently the Clearblue ones arent that great and dont pick up the hormone as early as the supermarket ones do.
Try and leave it as long as possible before you test though (not easy I know)
I know I'm so bad I have tested early for the last 5 months ad each time been upset but otherwise I just find that the closer I get to my period the more excited I get!. I'm also finding it really difficult with it being Irregular before my DD1 I was always 27 days.
Try your local independent chemist, there is a test that you can buy which tests for the HcG hormone earlier than any other, I think it's called something like First Sign (not first response) and it comes in purple packaging. It costs about £3.99 for 2 tests.