This strange bee with an extended abdomen is a male "Cuckoo" Vestal Bumblebee, also called the Southern Cuckoo Bumblebee, one of 6 species of cuckoo bumblebees in the UK.
Previously the cuckoo bumblebees were put in the Psythirus genus, but have now been moved to Bombus with the 18-19 other UK bumblebees.
Unfortunately the tall container in which it was temporarily held to allow inspection did not allow the camera to get close enough for a clear, sharp, "macro" photo.
Above are female Cuckoo Vestal Bumblebees caught in flight, apparently searching the grass for a Buff-tailed Bee nest to invade. The one above left has parasitic mites on her back.
She too is a parasite, invading the nest of her host (the Buff-tailed Bumblebee), killing the queen and laying her own eggs which are then raised by the Buff-tailed workers.
The nest then dies out, since the cuckoo bees have no workers and contribute nothing to the host colony.
(The Gypsy Cuckoo Bee, Bombus bohemicus, that invades nests of the White-tailed Bumblebee, looks very similar.)