Trailer head height

Most horse trailers would be tall enough for a 15:1 its when your looking at the 16:2 + that you need worry.
eg
Equitrek M is 7 ' 3" as is IFW 506.
 
Why does "750mm above the withers" keep popping into my head? Is there something in the travelling regulations? Sorry but I keep seeing this thread but am too lazy to look up the question! Would it be on the DEFRA site? Anyone out there with nothing better to do?:D
 
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