Peper Harow may do it. They are based near Guildford. Bought my lorry from them and they are very good. They recommended someone to do the inspection for me, as clearly they couldn't inspect their own vehicle, so even if they don't do it they will be able to give you a name.
Aha! Found the one they recommended -
Taylor Commercial Repairs Ltd
01483 276608
Please contact Wayne Henning of Henning Horseboxes. He takes on pre purchase inspections of horseboxes and is excellent. His number is 07762 792996 He has a Facebook page with videos of him checking boxes. Some of them are shocking and you wouldn't travel in it yourself let alone put your horse in it.
