A letter of Invitation is a proof that your paper submission and registration application are accepted by the conference committee board. It perhaps could help with your visa application. However, it does not guarantee you a visa.
The invitation leter will be issued ONLY after you have registered and full payment has been received.
The UK visa is processed in your country of residence prior to your departure. Before you apply, you must check if you need a visa and what type you need. Depending on your nationality, you might not need a visa. Your application must be approved before you travel.