The picture shows the boat at Poole headquarters, it's on a road trailer for rapid deployment to its point of use where it would then usually be transfered onto the local launch trolley, however boarding boats do start with a BB and this number B-10D has confused me for some time.
It's not a capsize training boat as it would be D-000-C and in the same fitment as a coastal D-class.
The best guess I've heard is it is a flood rescue D class kept at Base 10.
Apologies for going all geek there
Avon Searider SR5.4 "Challenger"
Ex RNLI Y-55, Ex RNLI D-???, Ex RNLI D-486
Ex RNLI 50-001 "Rotary Service" = www.50001.org.uk