Back in my day, you could actually get a glass of Guinness afterwards (perhaps to restore lost iron after donation but, not being an expert in such matters, I can't say for certain).
Needless to say, this was not the only reason that I chose to donate... 
Funny you should say that but for the first time in 60 donations my iron was a little low and they had to take a second reading after the 'sink' reading. I was fine to donate (it has to be above 150 or so) but now they tell you to avoid alcohol afterwards.
Obviously I ignore that suggestion but I will say that as I get older (approaching my 51st Birthday) it does make me rather tired if only for a few hours.
But well worth it. Apparently every pint donated saves 3 people.
Do it chaps. You know it makes sense. It could be your family you are saving or, heaven forbid, yourself.
And they make a lovely cup of tea.