[Nyororin]

I think you`re sort of missing out on what is being said. The stamp they grant you IS a visa. Is it granted through a program that allows you to automatically receive one when you enter the country? Yes. But that doesn`t mean it`s not a visa. In official documentation it is referred to as a visa. "Tourism visa" is the proper term.
You just don`t have to apply for it ahead of time if you`re from certain countries.

Anyway - I apologize for the mistake in my former post. I sort of skipped over reading that you were from the UK. So yes, you can legally stay for 6 months without changing your visa status (tourism).

Countries with a 90 day limit can also legally apply for an extension, and in that case what I said in the previous message still applies.