After my previous post I tried to search online for “UK bright green caterpillar”. The AI search summary came back with
Bright green caterpillars in the UK can include species like the Elephant Hawk-moth caterpillar, which is chunky and has purple and white stripes, and the Cinnabar moth caterpillar, known for its distinctive black and yellow stripes. These caterpillars are often found on various plants and can vary in appearance as they grow.
So not the Cinnabar moth at all then?