I’ve never had an issue at St Pancras. Whilst they certainly keep the gateline closed to incoming at certain times, if you slip through and find somewhere quiet to wait on a platform I’ve never had anyone bat an eyelid.
King’s Cross seem to be occasionally closing the gates to incoming for the suburban platforms recently, and I have even been pestered by staff there whilst waiting on platform 10. This is an unfortunate development as King’s Cross has always been quite chilled especially since the new concourse opened.
As far as the original question goes, Skegness can be added to the list. Exmouth used to be as well, but not sure if this is still the case. Totally don’t understand why Exmouth ever worked like that.
I think Bordesley and perhaps Polesworth are kept locked.
Then there’s Acton Bridge where a local resident didn’t like certain types of people using the station, though we don’t seem to have heard of trouble here for a while now, so perhaps the troublesome couple have moved away now.
Another old one was Lelant Saltings, where they at times seemed to operate a queuing system, which involved having the platform cordoned off with a tensa barrier. I remember bemusing the staff there when to circumvent this arrangement in order to take some photos of the station we arrived by train and remained on the platform - the member of staff wasn’t bothered, but couldn’t understand how we’d managed to get on the platform! This is all history now as the park & ride has moved, but there may still be queueing systems at St Ives and possibly St Erth at very busy times. St Ives seems to have gained some shouty staff in respect of the yellow line, which is rather daft at a small station where trains approach at crawling speed.