Alanaslan - as long as the horses don't take the horsebox over it's gross weight (the most it'll ever be in the UK is 3500kg), and as long as the trailer isn't heaver than the maximum towing weight of their 4x4, then the 'combination' as it stands should be legal.
Whether the drivers you mention have the correct licence to tow a trailer is another matter however...
I'm not sure how true it is, but I believe that in a situation where the gross train weight (car & horsebox) is over 3500kg, if the driver receives any form of 'betterment' from the purpose for which they are towing, e.g. winning a trophy or even just a rosette in the horse trials, then legally they should be using a tachograph.