Chrysolina americana (Rosemary Beetle)
Photo Copyright Royal Horticultural Society
Despite the name Chrysolina americana this species is native to Southern Europe, and has been recently introduced into the UK, where it can be a minor pest of Lavender and Rosemary plants. The Royal Horticultural Society has a page about this beetle that has lots of useful information.
I have seen this beetle in the vicinity of Clapham South London Undergorund station , and outside of Darwin Centre phas 1, Natural History Museum, London.