The participants coming from outside the UK and requiring visa information should visit the web site www.ukvisas.gov.uk, 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.