No offense taken Josephine. I have been there and with all my issues, I know how bad it feels when one wants to be pregnant.
You are right about progesterone being higher when pregnant. However, a CD 21 test (which by the way should never just be performed on CD 21, but at 7 DPO instead) would not be able to show you if you are pregnant.
For example, let's take a 30 day cycle and say the woman ovulates at CD 16. She should get the prog. test done at CD 23 (7 DPO). If she is indeed pregnant implantation would occur anywhere between 6 DPO - 9 DPO most likely. At 7 DPO the chances of there being enough HCG in a pregnant woman's body to raise progesterone levels are slim to none. Even if the baby had implanted early at 6 DPO that would only give you an HCG level of 5-10, not enough to trigger a high progesterone level. Higher than normal progesterone when pregnant wouldn't really be apparent until at least 14 DPO.
To answer your question I wish I knew more about UK levels but I live in the US. In the US here are the progesterone levels you should expect to see:
at 7 DPO -
at least 10 to show healthy ovulation
at least 15 if the cycle was medicated
at 14 DPO -
a level of 25 in a pregnant woman (with a bare minimum of 10)
Hope that helps a bit!