Clarification on Permitted Business Activities and Strictness of UK Visitor Rules

Hi all, I’m a bit confused about the UK Standard Visitor visa rules.

If someone is applying for a short stay for tourism or a business visit, how strict are the rules around what they can actually do while in the UK?

For example, what counts as permitted business activities versus something that would require a different visa? And are there common mistakes people make when applying under the visitor category?

Would appreciate any clarification from those who’ve gone through it. Thanks!

Has anyone from a non-visa-exempt country actually used this visa for mixed-purpose trips (tourism + freelance checking in)?

I was informed of a relevant incident regarding a colleague who traveled last year.