Having worked in the British civil service myself, I can tell you that most grades dealing with members of the public are officers of some level, for example administrative officers, executive officers, higher executive officers etc. Sometimes they have a specific job title relating to the service they provide, but unless you knew what that was "officer" is a pretty generic term that covers most roles and would be understood.
So you might say:
I went to a local government office and spoke with an officer.