Switching from a tourist visa to a fiancé visa while you are on holiday in the UK can be a complex process and is generally not allowed. A tourist visa (Standard Visitor Visa) is intended for temporary visits for tourism, family visits, business meetings, or other limited purposes, and it does not permit you to marry or form a civil partnership in the UK.
If you are in the UK on a tourist visa and wish to marry or enter into a civil partnership with a British citizen or settled person (someone with Indefinite Leave to Remain or a settled status), you will typically need to return to your home country and apply for a fiancé visa (or marriage visa) from there. This visa allows you to enter the UK for the purpose of marrying your partner.
The fiancé visa application process typically involves meeting specific requirements, including proving your genuine relationship, demonstrating your intention to marry within six months of your arrival in the UK, and meeting financial criteria. After the marriage or civil partnership, you can then apply for further leave to remain and eventually settle in the UK if you meet the necessary conditions.