That's because you're a UK citizen. You are subject to the laws of your own country as well as the "law of the land", i.e. the law where you are committing acts.Most laws are territorial: you can only break them in the territory of the country, others aren't. The UK, btw, can prosecute you for paedophile sex acts commited abroad, as for FGM. Hacking is considered to be carried out in the territory of the server regardless of where the hacker (although it can be both).
If you weren't a UK citizen or committing acts in the UK, the UK would not be prosecuting you for FGM, paedophilia or most other crimes under UK law. Unless the victim were a UK person or entity.
So your statement that everybody is subject to the laws of every country, needs some heavy qualification added. Because otherwise it's just not true.