It is the responsibility of the individual conference delegates to ensure that they apply for an appropriate visa for their travel to the UK in plenty of time. It can take several weeks to process visa applications.
All individuals entering the UK are assessed against the Immigration Rules to ensure they are coming for a genuine purpose. Business visitors can enter the UK to attend conferences and are expected to provide information on what they will be doing in the UK as well as details about the host organisation. ARCOM can provide a letter of invitation for those delegates who have registered to attend the conference and have paid their conference fee in full. Please contact the ARCOM Conference Secretary.
If you have any questions, you should contact the Home Office. Information is also available from the Home Office website: https://www.gov.uk/browse/visas-immigration.