phone your local BS venue, they often have tickets to ride for the first few people who ask for one. Rowallan have tickets to ride for all classes, and SNEC offer for the riders first class up here so will probably happen down with you.
If you're location's right then either sparsholt or pachesham (or maybe both..? I can't remember where I saw it) have been giving free tickets to the first few who ask for them.