Dear GreenMark,
As far as I know you do not need to fill in any application forms. You just need to follow the things that have been stated in the invitation letter.
Make sure that you have recent photos, the invitation letter and all supporting documents with you. Take supporting documents as much as you can.
If you are not living in London, then my advice to you is to travel to London 2 days before your interview date (to avoid unpredictable things such as whether, ticket cancelation, ...).
If you had any question then ask.
Please share your interview experince with others. My interview will be held in London as well.
Do you use your own assets or you have sponsor?