This issue was troubling me and although there were some posts on here, they were all a few years old, so today I rang the NHS Blood and Transplant Service.
As expected I cannot donate blood or marrow, but this is the response from the Organ Donation team (refers specifically to ET, not other MPNs):
"It is still possible to donate some organs and tissue even when you have cancer. There is a small risk that Infections and other diseases can be transmitted from the donor to the recipient and this risk is made greater by the drugs taken by the transplant patient to stop their body rejecting the new organ. However, the risk that this will happen depends to some extent on the type of cancer, the treatment that has been received and the length of time since the disease was diagnosed and treated.
"There is no reason why you cannot register as an organ donor if that is your wish and the medical staff at the time can access your situation and decide on which organs are safe to use for transplantation. The medical team will always consult the family of the donor to check up on medical and lifestyle histories as well as reading their medical notes so will be aware of any conditions".
Happy now!
Mel.