The participants coming from outside the UK and requiring visa information should visit the web site www.gov.uk/government/organisations/uk-visas-and-immigration, which is operated by the UK Home Office and Foreign and Commonwealth Office.
This site features an on-line Visa Enquiry Form enabling you to find out whether you need a visa or not. If you do need a visa it will provide you with links to the forms you need to fill out and contact details for your nearest British Embassy, High Commission or Consultate.
If you need a letter confirming your participation to the conference, then you can request it from the Registration Tool. However it will only be issued after a registration and payment has been received, and there can be no exceptions to this.