I, too, thought the idea of secret Santa is that you bought one gift only for the person whose name you drew out of the hat. If you’re all buying 10 gifts then doesn’t that mean that you all will receive 10 gifts, you just won’t know which person bought which gift you received? (I’m asking this because you mention £100 spending).
Anyway, back to the posters query - if you prefer to donate your £10/£100 to a charity, just say that’s what you would like to do this year rather than be part of the secret Santa so that way you won’t receive a secret Santa gift either. Instead only 9 of them will be participating.
We used to do secret Santa at work, but there was no pressure to be part of it - it was up to the individual. Those participating bought just one gift at a pre agreed budget.