Even if she wants to cut it to hock length, the horse will be FINE.
For all y'all having the vapors about her not being able to swat flies, may I remind you it's WINTER in the UK, just like it is here in the US?
I know that one of the selling points of GVs are their over abundance of hair, but unless the horse is some high-falutin' expensive show nag who'll be on view by the Queen of England, what the heck difference does it make?
Tempest in a teapot, for cryin' out loud.
OP, if you want to cut your horse's tail instead of spending hours trying to get the danged thing clean and burr free, go ahead. The sun won't fall out of the sky, and your horse isn't going to drop over dead from having a shorter tail than the 'approved' length.