Is it not more a case of they stick together and dont want to fraternise with the natives. Having known a few police officers over the years they did all tend to socialise together rather than meet up with outsiders in the pub even as young men we noticed that. I just assumed they were told not to be too friendly with non-officers in case of being led astray or other such suspect possibilities.
I can also see by your avatar that you do indulge in the water of life. A man after my own heart sir altho not so much the peaty type stuff.
Dave