Your best bet is to look at all the websites of bands in London, and those outside that are easy to get to. I believe Regent Brass have a vacancy in their main band and I've found the best way to contact them is via Instagram but they do have a contact form in their website. They'll be in the first section next year. If you want top flight contact Fulham or Zone One. I don't believe Fulham have a vacancy, and Zone One is a project based band as opposed to a regular one. If you are drive there are loads of First Section bands outside of London so worth contacting them also. Use the band map on Brass Band Results if you need to know their general location.