Hi gooner, it's a lovely thought of you to spoil your mum. I think she will be made up just for the fact that you are trying to make her happy, regardless of how big the gift is.
It depends on what your mum really likes: maybe vouchers in a shop that she really likes, a nice engraved watch or neckless, a football season ticket, or you could buy her tickets to see her team playing abroad, if they are playing abroad anyway
....I know English people are too into football.
Maybe pay for "Scuba Diving" lessons, if she is interested. You should know her better. But the idea on its own will cheer her up