Similar to an AIM-9(N) Sidewinder (even if the fins seem to be different from any known Sidewinder or even Atoll model…) the missile in the hand of the javelin thrower could be the means the notorious anonymous painter has chosen to use to condemn the militarization of London in advance of the Olympics.
Indeed, along with Eurofighter Typhoons on QRA (Quick Reaction Alert) at RAF Northolt, several more assets enforcing a No Fly Zone around London while the HMS Ocean is anchored at Greenwich, the large military machine put in place by the UK to protect the Games sees also Rapier SAM batteries deployed at sites around the capital.
Some of these sites have been controversial as they have been set up on roofs of apartment buildings and many locals have protested at the missiles proximity to their homes.
This could have inspired Banksy for his mural. Although he took the license to use an IR-guided air-to-air missile instead of a radar-guided SAM (Surface to Air Missile).