They are many Tortoise Beetles in the genus Cassida. All look more like scale insects than beetles,
especially when they perceive a threat and lock down on a leaf or surface like a tortoise.
They are leaf beetles. Cassida vibex, shown on this page, grows to 6-8mm length.
It looks similar to C. viridis, the Green Tortoise Beetle,
but the later is always green without the dark markings of vibex.
The Thistle Tortoise Beetle, C. rubiginosa, is also similar.
Above right shows the beetle underneath the shell.
In the UK, they are found mostly in the southern half of England.