Fredtransit spotted this on ebay, would have a high enough limit to tow a horsebox, its a classi, tax free and would be reasonable economical, if a bit slow with a trailer on the back.
cgi.ebay.co.uk/ford-transit-mk1-tax-free-4-berth-1970-modern-body-/320555873177?cmd=ViewItem&pt=UK_Campers_Caravans_Motorhomes&hash=item4aa29e7399
cgi.ebay.co.uk/ford-transit-mk1-tax-free-4-berth-1970-modern-body-/320555873177?cmd=ViewItem&pt=UK_Campers_Caravans_Motorhomes&hash=item4aa29e7399