If a passenger gets so drunk they are prevented from boarding then the bar in the airport gets fined for continuing to serve.
The difficulty is 1. proving which bar, and 2. most guests buy duty free, open the bags and decanter it into soft drink bottles so harder to detect.
If a guest is seen opening those sealed duty free bags they have the alcohol removed from them without refund. If they've already drank it they can be prevented from boarding and fined.
The issue is, it costs money to pursue the fine and usually its not worth it.
Passengers can get banned from flying for X number of days from that airport though which is good - prevents them from being removed on the Monday for being too pissed, rebooking for Tuesday and coming in causing a scene.
There are airport police but usually they are not in the terminal unless on a 'walk around' and can easily take 20+mins to attend a call out, usually longer.
Like a previous poster said it can be very intimidating for a large group of drunk people to be carrying on, knowing that to remove them from the airport can take 1.5-2hrs after the check-in point. It's not like kicking someone out of a highstreet bar and turfing them onto the street.
They need to go through immigration like any passenger returning to the UK as they've entered the airport, this isn't a simple or quick process.