A few years ago, my husband got locked in at Streatham (south London). He'd caught the last southbound train, arriving around 00:30, then got a phone call when he was still on the platform. Maybe 15 minutes later he came upstairs to the ticket hall to find the doors locked. After failing to get through on a Help Point or find a number for Southern, he apologetically called 999. They were very nice and confirmed someone with a key would be sent - but it was over an hour in the cold before he was rescued.
Streatham station definitely needs to be locked up when not in use, but stations really should have functional ways to contact staff in an emergency.