as a foreigner, you can’t even volunteer at most organizations without it technically breaking the law. There are very stringent requirements. I’ve been pulled into secondary by immigration and questioned and lectured for well over an hour on multiple occasions. Anything that could earn money either for you or a local, would be considered illegal work. So as pet sitting is a paid job for some and, if we weren’t doing it for free, would be a paid job for a local person/company, they would consider it illegal work. I was told by them that even writing a blog post that earns money while I’m there would be considered illegal. The UK is, by far, the most strict country I’ve dealt with in 13+ years of working as a nomad (ETA, I’m from the US, so I don’t have to worry about their working laws when I’m there, though they have harassed me on reentry more than once). As for where to read more, I’m sure it’s detailed somewhere on the UK immigration website.