Something is off here. The OP seems to know rather a lot about what might be the outcome of fare evasion, and BTP indeed don't normally work like this.
I suspect there may be aggravating factors that have been omitted, but going on the basis that we've been told the full story - If you have any doubt about the legitimacy of the officer (assuming it's BTP), I'd just call 0800405040 and say that you've been contacted by (name of the officer) asking you to go to (whichever) police station and you'd like to check they are a genuine officer. They should be able to tell you if they are pretty quickly.
I suspect there may be aggravating factors that have been omitted, but going on the basis that we've been told the full story - If you have any doubt about the legitimacy of the officer (assuming it's BTP), I'd just call 0800405040 and say that you've been contacted by (name of the officer) asking you to go to (whichever) police station and you'd like to check they are a genuine officer. They should be able to tell you if they are pretty quickly.