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From memory, the corners of the pronotum of C. viridis are more rounded than in your specimen. For example
What plant was it on?
I think I'm right in saying it was on comfrey. I may have other photos showing other aspects more clearly - which areas would help?
Underside might help. Close-up showing puncturation on elytra? On the whole, I'd rather have Cassida specimens under the microscope, but someone else might be able to do it from photos.
I have another photo, perhaps showing further details useful for id. Don't know how to add it other than in a new observation (27th Aug).
This is Cassida rubiginosa (thistle tortoise beetle) the pointed corners to the pronotum are clearly visible. These are much more rounded in C.viridis (as J Bratton points out) the slight 'rusty' mark on the scutellum is variable but just visible.
Lat/Lng: 52.7494, 0.078
OS grid ref: TF403188