Certainly a valid point and being in that situation might cause you to think that one should be able to screen recipients. But I think it goes against the principle of helping whomever is in need without judgement. NHS, unlike private alternatives, treats regardless of underlying causes to the conditions (to the furthest practically possible extent) and thus once a donation is agreed, it should go to the best match with highest need, regardless of moral stance. This would otherwise open up for discrimination in healthcare systems.
If you're happy to give, than give the gift without judgement.