The reply from KMS is only partially correct.
The FI must also have completed a CPL standardisation training course followed by a flight test with the CAA, after which he is an approved CPL instructor as far as the CAA are concerned.
He must then be approved by the Head of Training of any FTO for whom he wishes to fly, the FTO course itself must be approved and machines must also be approved on a specific basis.
The training must also be carried out at the nominated base of the FTO.
Bet they could think of something else to approve if they really tried........