What are the height regulations for a lorry/trailer?

[IAmEllie]

Question as in title.
Kita's owner has just bought a small second hand lorry which has previously been used for transporting vegetables(!) although apparently so they say, they have had 2 shire horses in it.
It has 6 months tax and mot on it, but it needs a lot of work doing to it before a horse can go in it, including sorting the floor out. We were just wondering what the height would need to be, with them aim to be transporting a 15.3 horse. We measured the height tonight, and as it is currently, the height from floor to roof is 83 inches (1 inch shy of 7ft)

Any help greatly appreciated, or if anyone can possibly point me in the direction of where I can find the information.

 

[Iron Maiden]

I don't think there are height 'regulations' as such. My wagon has 7 ft 5 headroom and takes a 16.2 horse with quite a long neck easily. You might be OK with under 7 ft, but it sounds a bit tight to me. Also the floor might well need reinforcing, which could bring the headroom down. YOu could check out the headroom on trailers designed for horses of different sizes to give you an idea of what you might be aiming for