Well, you linked to his registration entry but that's not donations. In fact he doesn't have any donations so maybe that's why you were confused. If you look for a political entity which does have donations, like Reform, you can see £50M over the years since it was formed (then as the "Brexit Party")

And it's not true that "anybody" can donate. People can donate if they'd be allowed to vote in the UK, which currently includes people who don't live here and so don't care what happens but that might change to some extent. Non-people such as businesses can donate if they are legitimate UK entities with UK registration or whatever as appropriate (so e.g. a UK business you own for no purpose but to fuck with UK politics wouldn't fly, but if you've got a money-losing Californian Wine Importer that's a fun hobby you can make that loss $5000 bigger by having them them donate $5000 to Reform if you like)