Freelance jobs vs pocket money

This will probably sound stupid but im so clueless on this stuff.

I live in the U.K., when do you have to register as a freelancer?

If im working part time to clean my sisters house for a few hundred pounds a month surely I don’t have to declare anything right?

It’s not a business and I’m not early nearly enough for taxes. It’s basically just a bit of extra money.

What are the rules of this stuff? At what point do you have to declare this stuff so I don’t get into trouble.