I'm sorry you're so annoyed @
I like Skol but I can't believe that the National Blood Service is really 'taking the p1ss' or treating donors 'like commodities'. It's part of the NHS and is struggling with cuts in funding just as much as other areas. The people who staff the donor sessions have to put up with abuse sometimes from people who can't be accommodated when they wish to be, but it isn't their fault that this happens. No one goes to their GP surgery without an appointment expecting to just be seen there and then, do they?
There's always the fact that a surprising number of people make appointments for things and then just don't turn up - maybe they forget, or maybe they can't be bothered - then perhaps a drop-in donor could be slotted in. Depending on where you live there might be other sessions where it would be possible for you to make an appointment to suit your circumstances, but it would be a great shame if you just stopped giving blood altogether because of this.