I have to say I haven't done so for years, having got out of the habit and no longer being sent a reminder but finally I managed to co-ordinate some free time with the Blood Transfusion service running one of their regular donations sessions at the Meridian Centre and went along to give blood. When I worked at Amex it was easy - they turned up a couple of times a year - boards were placed in reception areas letting you know they were coming and you signed up.
It's a relatively painless (no honestly) process - requires about an hour of your time and obviously many people wouldn't survive without transfusions. They take slightly less than a pint - not a whole armful as Tony Hancock's famous Blood Donor Sketch goes and it's not just the blood that gets used.
If you can donate then do. You can find your nearest blood donation session from www.blood.co.uk