To be a CTSFO officer you first have to get in the police and complete your two years probation and probably get a few more years street experience in and prove yourself as an operational police officer. You then have to be selected for firearms training and pass a basic firearms course with the Glock pistol.
I am not sure of the system now but in the Met you then had to serve on an Armed Response Vehicle team reacting to fire arms incidents anywhere in London. They then got selected and trained for a Specialist Firearms Officer Team, now CTSFO.
If you apply to the police saying you want to be a CTSFO it is doubtful if you would be accepted. It is an ultimate ambition to have as you go through your probation and get further street experience.