I would go for a 3.5t horsebox van.
This sort of thing (first one that came up on eBay, this one is apparently sold) https://www.ebay.co.uk/itm/276482373554?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=un4MMHv6SY6&sssrc=4429486&ssuid=5lEspz7VR4C&var=&widget_ver=artemis&media=COPY
Look in the back of horse magazines, or horse forums for listings.
They hold their prices really well so you shouldn’t lose much on it.
I’m sure that there is also some special rate of VED for horseboxes, and insurance was insignificant.
It also means that you will not be running another car that is bigger than it needs to be when horses are not involved.
My wife recently sold her 28 year old rust bucket of a Sprinter that we’ve had for 10+ years for about £5k. I think that we only paid £8k.
It was an absolute wreck that stank of horse piss, but reliable for the occasional horsey use that it was needed for.
It also helped on several house moves, was slept in at festivals, and acted as the stage/nightclub at our wedding in a field.