Under the present system you need to meet the requirements of Standards Doc 10. As the FI revalidation forms no longer contain a level i.e average; above average etc (They did pre 1999 but no longer PC) then you are required to apply to the CAA and then attend a selection interview where the panel will determine if indeed you are above average. Following the interview and subject to a recommendation, you complete a 2 day course with a FIE and then take a test with an FIE who was one of the members of the selection panel.
You do not need to be sponsored by a FTO, but the approval of the individual is tied to a nominated FTO.
Under EASA things may well be different, if you meet the experience requirements and pass a test with an FIE then the CAA will have to authorise you without adding their own requirements. The standard will inevitably lower and there will be more FIC instructors. The way of Europe!