Tony,
Your concerns are perfectly justified. Actually, I was worried about the same thing but am going to keep my Ginkgo despite its blood thinning properties. Let me explain why. The most famous blood thinning medication - Aspirin (and in fact some of the other drugs in its class) - has been found to reduce the tumor size and the number of tumors in certain types of cancer. So, the blood thinning properties per se do not necessarily enhance the tumor growth, quite the opposite has been found with Aspirin.
If you are still concerned and would like to stay away of the St. John's Wort because of its blood thinning potential, I would like to recommend Rhodiola Rosea. I researched it and it is available in the UK. It is supposed it to be a mild anti depressant among other things. I have no idea how it compares to St. John's Wort and I have started taking Rhodiola for a different reason - increasing stamina. Let me tell you - it works! And fast! My only (and big) regret now is not taking it sooner after surgery. It is not a blood thinner as far as I know and it does not raise your blood pressure like Ginseng. While on it, my days are no different than those before surgery and I don't need naps. That means I can move more which will make me recover faster!
Good luck to Jessica with her treatement. Thanks for posting.
Eve