I'd recommend a Ford Transit Connect. Lots on the used market, which means you can afford to be quite picky when it comes to age and condition. It's also available as a short or long wheelbase, with a low or high roof. The sliding door on the passenger side should make it easy to get your diving equipment in and out. A removable mesh bulkhead is also standard which means you don't have to worry about equipment sliding forward into the passenger area if you have to make an emergency stop. For our full review and used buying guide, see: vans.honestjohn.co.uk/van-reviews/ford/transit-con.../

A Vauxhall Astravan might also suit your needs, with lots of load space and a braked towing capacity of 1.4 tonnes. It doesn't get the side sliding door, but it is one of the best vans to drive, owing to the fact it's basically an Astra estate with the rear seats removed: vans.honestjohn.co.uk/van-reviews/vauxhall/astrava.../