If you are a non-visa national you do not need a visa to come to the UK as a visitor.
However, some non-visa nationals prefer to obtain a visitor visa before they travel to the UK so that there is no risk of their being refused leave to enter the UK.
However, if you are a visa national you will always need to obtain a visitor visa to visit the UK.
A UK visitor visa entitles you to visit any part of the UK and Islands, which include England, Wales, Scotland, Northern Ireland, Channel Islands and Isle of Man.