The participants coming from outside the UK and requiring visa information should visit this website, which is operated by the UK Home Office and Foreign and Commonwealth Office.
This site features an online 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 Consulate.
If you need a letter confirming your participation in the conference, then you can request it from the Registration Tool. However, it will only be issued after the registration and payment have been received, and there can be no exceptions to this.