So we are talking about a 7 seater here. TBH, i think nellyjack's suggestion of just renting a minibus makes most sense here. With that budget, for the space requirements you'd be looking at a big MPV (a Zafira or similar would be no use, as you'd need to send the luggage seperately), probably diesel, or possibly an actual minibus. It is not going to be very young for that money, or if it is, it will have done a lot of miles. So the running costs could easily make it more expensive than hiring as and when needed.

Some of my mountain biking friends have what are in effect vans with back seats
and back windows.

VW Transporters, Vauxhall Vivaros, Ford Transits, Mercedes somethingorothers too.

Whether a usable one of those could be bought in budget is another question I suppose, but they are cavernous.

Problem here is that the OP needs 7 seats, whereas crewcab vans, unless it is one of the really big ones, will be 6 seaters, 3 in the front and 3 in the back.

There are certainly minibuses available within budget, though as to whether or not it would be a reliable buy?, that is debatable. I did have a look on Autotrader (vans), and found this Transit Tourneo,

www.autotrader.co.uk/classified/advert/20190613898...1

which doesn't seem too bad?