The EN states this under
The contravention has already been dealt with:
Please supply proof that the Penalty Charge has already been paid or that a warning has been issued by police/traffic warden/civil enforcement officer.
The law states this @
https://www.legislation.gov.uk/ukla/1996/9/section/9/enacted9
Penalty charge not payable in certain casesNo penalty charge shall be payable under this Part of this Act in respect of any conduct where—
(a)a notice is given to the driver under section 54(2) or (4) of the [1988 c. 53.] Road Traffic Offenders Act 1988 in respect of a bus lane offence constituted by that conduct; or
(b)notification of an intention to prosecute the driver in respect of such an offence is given by the Commissioner of Police of the Metropolis or of the City of London to the council of the borough in which the alleged offence took place before the expiry of the period of 14 days, beginning with the day on which the alleged offence takes place.