See here:
http://www.ngdt.co.uk/images/stories/pdfs/becoming_a_sperm_donor.pdf
Look through the list of clinics, paying particular attention to their email addresses. (Those clinics within the NHS usually have 'nhs' as part of their email address).
From that document:
"The British Fertility Society guidelines state that all healthy men who are aged between 18 and 40 are eligible to donate.
They also need to be free from any serious medical problem and disability and with no history of congenital, family or hereditary disease.
Information will also be required from you about your medical and family history. Potential donors will also need to have blood tests, a general medical examination and produce a sperm sample at the clinic".
Those rules would appear to be the same across
all clinics, irrespective of whether they're run by the NHS or not.
Chris