If you have a degree at the highest academic level (PhD or higher), you have been working in your field of expertise for a long time and are not going to occupy a permanent teaching post at a British university, you can apply for a UK Standard Visitor visa valid for up to 12 months.
However, an application fee will be higher:
£190, if you apply through a visa application centre in your country;
£993, if you come to the UK on a Standard Visitor visa valid for up to six months and decide to extend your visa for up to 12 months in total.
As you can see, if you want to minimise costs, it will be a good idea to apply for a UK Visitor visa valid for up to 12 months before your visit, instead of paying extra costs for staying longer when you are already in the UK.