hellojulie, I don't think it is clear what the Passport Office wants to be entered ..... and I don't think it matters much anyway!
That is, you need to enter the number of your Certificate of Naturalisation, and indeed supply the certificate with your passport application. Clearly the Passport Office can see you are British.
If it helps ..... at what instant in time did you become British, and where were you? It was at your Citizenship Ceremony, so totally logical to enter the name of the city/town where that ceremony took place.