I am English and although I gave blood many times before I moved to the US, I am not able to give blood here in case my blood has mad cow disease (?) otherwise I would donate. It is painless, takes a short time and is a very easy way of doing something very worthwhile which gives you a "feel good" feeling of helping out. After all, you never know when you or a loved one may need someone else's blood.
My daughter has also tried to donate, but although she has never lived in England, she has not been allowed to donate either because the number of times she has visited the UK has put her blood "at risk" too. There are so many people whose blood is not wanted that for those who can donate, it is doubly helpful to do so.