You must obtain a visa before you can come to the UK to work or do business.
Generally, you must apply in one of the following categories:
Tier 1 of the points-based system - for investors, entrepreneurs, exceptionally talented people and recent graduates of UK universities
Tier 2 of the points-based system - for skilled workers who have been offered a permanent job
Tier 5 of the points-based system - for temporary workers with a job offer, and participants in the Youth mobility scheme
domestic worker in a private household
representative of an overseas business
The Working in the UK section contains more information.
You can apply for a visitor visa if you want to come to the UK for up to 6 months as:
a business visitor
an academic visitor
a sports visitor
an entertainer visitor
a prospective entrepreneur