I would leave it. Long manes look good in reining, and there's no reason to shorten it if you're going to do a running braid for english.
The only problem is WP. I supposed you could band it but it might look silly banded so short. Maybe you could like fold it in half and band it to create an illusion of a short mane? :p Idk, good luck though!
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman