(Acanthosoma haemorrhoidale)
This is one of the Shield Bugs, so called because of their resemblance to heraldic shields. "Bug" is often used as a slang term for insect, but these insects belong to the order Hemiptera, the True Bugs. The species shown here feeds mainly on the fruits of the hawthorn tree.
I came across a nymph of this species on the same day:
NS3677 : Hawthorn Shield Bug.
For further information on this species, see
Link (at the British Bugs website).