Also, it's a green shield bug,Palomena prasina. It is commonly mistaken with the larger North american stink bug,Acrosternum hilare. The green shield bug is however very common in Britain , so I am almost certain it is one.
It may be mistaken for the birch shield bug, which highly resembles it and has the same distribution, being all over Europe. It has a bright red "X" like pattern like the Hawthorne shield bug. Here is a picture : http://goo.gl/Pd4fk