You may need a visa to travel to London, so check the regulations and apply in good time if a visa is required. Visit the official UK government website: https://www.gov.uk/apply-to-come-to-the-uk.
Your visa invitation letter can only be accessed after your registration. Once you register, your visa invitation letter will be autogenerated in a downloadable form. Download this letter and use it for your visa application letter. Please register early so that you have enough time for the visa to be processed before you arrive in London.