Afraid you cannot just train up to become a freelance CELTA trainer.
The Cambridge Trainer in Training Guidelines state:
"A prospective trainerís application to be a CELTA trainer must be supported by a CELTA centre. The prospective trainer must be working at the centre where the training is to take place, or working for another CELTA centre and must be recommended for training by either the training centre or their own centre. They must be undergoing raining for employment at their centre where they must teach a minimum of three courses so that the training can be consolidated."
If you are working for the BC, then you may need to transfer to a centre which offers CELTA courses already.