It can be a little confusing when they are effectively the same vehicle, but it all comes down to vehicle classification. Most kombi vehicles are classed as M1, which makes them dual purpose vehicles. These have the same speed restrictions as a car. See here: www.gov.uk/speed-limits

For the full classification of dual purpose vehicles see: www.gov.uk/government/publications/car-derived-van...s

Most vans (including double cabs) are identified as N1 or N2. This deems them to be commercial vehicles, which means your Ford will need to be insured as a van. Your Transit double cab will also fall under the speed limits for vans. This means you’re limited to 30mph in built up areas, 50mph on single and 60mph on duel carriageways. Motorway speeds are the same as cars, 70mph, unless you're towing a trailer, which lowers it to 60mph.