Turnout is not marked in a showing class. Only the BSHA and CHAPS(maybe?) would allow you to wear a top hat anyway the rest all stipulate a strapped hat to be worn.
BSPS is the main affiliated body for pony showing. The best place to find affiliated novice classes would be their area shows. Plenty of 15 handers jump in the intermediates, so there will be no problem regarding height.
For novices- 1m, restricted- 1.05 and open- 1.15m, they can add 10cm for...