Hi J,
It's hard to know exactly what kind of caterpillar you have there because I can't really see both ends.... and I'm not so familiar with bugs in the U.K. But these 2 websites might be of help to you.
Oh, and don't panic! Most caterpillars are easy to deal with.
https://www.naturespot.org.uk/...
https://www.rhs.org.uk/biodive...
