The permitted purposes are set out in Appendix Visitor: Permitted Activities. In broad terms they cover tourism and holidays, visiting family and friends, business meetings and conferences, private medical treatment, and short courses of study of up to six months at an accredited institution. Certain permitted paid engagements are also allowed in narrow, pre-arranged circumstances.
What the visa does not permit matters just as much. You cannot take employment with a UK employer or run a business based in the UK. You cannot sell goods or services to the public, study long term, marry or register a civil partnership, which needs a separate Marriage Visitor visa, or draw on public funds. You also cannot use repeated visits to make the UK your main home.
Not everyone applies the same way. If you are a visa national under Appendix V, you must obtain entry clearance as a Standard Visitor before you travel. If you are a non-visa national, you may instead need an Electronic Travel Authorisation, a lighter and cheaper online permission rather than a full visa.
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