Citizens of many countries and in particular EU-citizens do not require visa for entering the UK for a series of business activities, which includes the participation to scientific conferences. To check the official and up-to-date information regarding entry in the UK as conference participant, check your status on the UK government webpage: https://www.gov.uk/standard-visitor/visit-on-business 

After registering to the conference (cf. Registration page), EuroGNC participants who require a formal invitation letter to apply for a visa may request it to the conference chair by filling out and sending this e-mail.