IIRC Witney has a Brass Band so get in contact with them. You’re not far out of Oxford either and there’s a couple of Bands there too. MoominDave here is a first class Bass Trombone player in Oxford (Kiddlington Brass), if you send him a private message then he might have some local contacts and suggestions for you. Edit. Abingdon is a bit of a trek for you (in Witney), but people do travel longer and/or further for the right Band for them ... you might possibly still have friends there and an understanding welcome.
Bass Trombone players are almost always in short supply, so when Bands return to playing you’ll possibly get an easier ride than expected. If you only read Bass Clef then Wind Bands are a good place to look too. For a Tutor Book I’d recommend an old edition of Tune a Day, books one and two.
You might have learnt on a G Bass Trombone, they’re pretty much no longer used so you’ll need a Bb/F Trombone.
Edit 1. A basic Bb/F Tenor Trombone will be fine for your use, for many years, as a Bass Trombone in a 4th (contesting) section and in a non-contesting Band too. As a returner, with a lot to relearn and maybe learn, anything bigger and/or more complex will be a lot more trouble that it’s worth and potentially unmanageable too.
Edit 2. You might know this already but Brass Bands usually have instruments that they’re happy to loan out for little or nothing, there’s often no need to buy an instrument of your own.