If you are a national from a country in the European Economic Area (EEA) or Switzerland you do not need a visa to come to the UK. These countries are: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Croatia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Irish Republic, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, United Kingdom. Iceland, Liechtenstein and Norway are not members of the European Union (EU) but citizens of these countries have the same rights to enter and live in the United Kingdom as EU citizens.
If you are from outside the EEA you may need a visa, depending on where you come from and what you intend to do while you are in the UK.
The UK Border Agency (UKBA) is the authority responsible for managing immigration and entry to the UK, including the issuing of visas.
You can apply for a visa up to 3 months before your date of travel to the UK. You should get a decision on your visa within 3 weeks; some countries differ - so check the guide processing times to find out how long getting a visa might take in your country.
PLEASE NOTE: According to UKBA website, conference participants are usually considered as "visiting the UK on business". It states: "You can apply for a Standard Visitor visa if you want to visit the UK for business-related activities, eg: you’re coming to the UK for a conference, meeting or training"