English: A Tortoise Beetle (Cassida rubiginosa) There are several British species of Tortoise Beetle (named for the way their casing extends beyond their body like a tortoise's shell). Tortoise Beetles, of which there are over 2000 species worldwide, are part of the larger Leaf Beetle family (which includes over 35000 species).

As for the beetle shown in the photograph, a small rusty patch around the scutellum (roughly speaking, the place where the three sections of its casing meet) and a thin brownish line between the elytra (the two rear sections of the casing) suggest that it is Cassida rubiginosa. [Michael Chinery, "Collins Complete British Insects"]

(In contrast, the otherwise very similar "Green Tortoise Beetle", C. viridis, is entirely green. There are several other Cassida species in Britain, but these either look noticeably different, are much rarer, or are southern in their distribution.)