If you're affiliated try www.bsja.co.uk and go to shows then diary. If you're not then you can always go on a ticket for £6 extra, but you can't get placed. Sorry I live basically the opposite side of England to you so no idea what goes on in your area!
there is also an unaffiliated pony club show at arena uk on 15th feb its open to members and non members. if you go on pony club website you will find a schedule. www.pcuk.org