You are right that a club can't simply say, "X supporter didn't show up, so let's claim the VAT back". HRMC won't have that. So agreed, what you have posted above is correct:
What the club could do is simply ask those not attending to gift their seat back to the club, and then the club can claim the VAT. A simple email from a fan, or a form filled in online would do it.
So, if anyone wants to explore it further, how it works is this: season-ticket holders who could not attend get asked to return/donate their match ticket or seat entitlement to the club. Saints could then treat the pro-rata value of that match as a donation rather than gate/ticket income, which means the club could reclaim or avoid accounting for VAT on that element. The club could also try to resell the seat.
Or you can encourage people to piss and moan, boycott and criticise the club...and harp on and on, entitled as ever.