Four types of visitors:
(a) Standard visitors, to undertake permitted activities Appendix Visitor: Permitted Activities
(b) Marriage/civil partnership visitors, seeking to marry or give notice to do so
(c) Permitted Paid Engagement visitors, experts in their field coming for paid work for up to one month.
(d) Transit visitors, entering the UK for up to 48 hours en route to another country outside the CTA (unless the Transit Without Visa Scheme Appendix applies. Appendix Visitor: Transit Without Visa Scheme.
As an reminder, work is not permitted unless expressly allowed by Appendix Permitted Activities