Sub-adult
Southern green shield bug Nezara viridula? The adults are much plainer.
From A Comprehensive Guide to Insects of Britain and Ireland:
"Although periodically accidentally imported in fruit and vegetables, this pest species is a relative newcomer to Britain's fauna, first recorded breeding in London in 2003 and since observed in several parts of London, East Anglia, Hampshire and elsewhere. Rare at present, but could easily spread. The species is native to Ethiopia, but present worldwide."