There seem to be a lot of changes to hat rules at the moment. The best way to keep up with what is approved and what isn’t is to visit the British Equestrian Trade Association website or the governing body website of the sport you’re involved with. There is currently a document on the BETA website which can be viewed here.
A few useful links:
British Equestrian Trade Association: www.beta-uk.org
British Eventing: www.britisheventing.com
British Dressage: www.britishdressage.co.uk
British Showjumping: www.britishshowjumping.co.uk
The Pony Club: www.pcuk.org
Riding Clubs: www.bhs.org.uk/enjoy-riding/british-riding-clubs
British Horse Racing: www.britishhorseracing.com
The showing world do not have one overall governing body, so always check the rules for shows at which you are competing.