Google the BMCA it’s a British trials club. They hold events from autumn to spring most weekends.
As for the cub and choice of bike it’s up to you but I wouldn’t touch anything other than a bantam to ride competitively. Everything else I’ve had like cubs and C15s are too heavy which really makes a difference on an old bike. Ignore the purists who still think it’s 1973 and everything should cost no more than 5 quid and never ride a trial.
I have a 500 rigid AJS I occasionally ride which is as close to original as I can get and they still moan.
Get a bantam, you won’t regret it!
Rich