Originally Posted by homonculus
In fairness I am not sure there is a limitation in the UK - I have never heard of one and I know the Blood Transfusion Service does not routinely say anything. I would be interested if anyone knows otherwise
In AIC 97/2004 (Pink 70), the CAA advise pilots "they should refrain from donating blood or plasma if they are required to fly within twenty four hours."
I don't know much about UK law, but it seems following this advice is merely an obligation in one's own interest, as it were.