I always thought PCSOs couldn't detain anyone. Turns out I was wrong - I just googled it and apparently they have the legal right to detain an individual for up to 30 minutes (until the police can arrive). However, they don't have the right to use any sort of force. So there's absolutely nothing they can do to stop you from merely walking (or more likely pedalling) away.
You will have caused an offense, though, so I suppose they could make your life unpleasant if they could find you again.
The law seems awfully anachronistic though. No doubt it was written when dim tyre dynamo-powered lights were de rigeur. Nowadays when I have 1600 lumens up front + several flashy things on the back I'm far more visible.
Of course, if you get knocked off your bike at night without pedal reflectors, I suppose a driver's defense would be that you were riding illegally.