Confidence: I'm as sure as I can be.
Notes: I'm assuming this is the right size: 2.5 to 3.5 mm long. These are flea-beetles, i.e. they jump. Metallic green-bronze and about 5 British species, this one stands apart by having contrasting colours on the top of the thorax and on the elytra: bronze-green on the elytra and pink-copper on the thorax. The larva feeds on sallows and willows and it is quite common in the South, thinning out northwards.