Security just want to see a boarding pass, they don't care who you are, they just need to know you're on a flight that day and don't have anything malicious in your hand baggage. Many people have their passport out thinking it will be checked, but it never is, yet they still think that is "passport control" and for some reason refer to it as such.
Indeed, security checkpoints at airports in the UK and much of Europe are only interested in seeing boarding passes. The same is not true in America, however, where ID/passport checks are mandatory at security.
Traditionally, boarding passes were checked by dedicated staff at desks and they still are at numerous airports (including Liverpool down the road). However, Manchester seems to have introduced a new procedure in the last year or so, where the actual security screeners check boarding passes as you are putting your belongings in a tray to be scanned. Gatwick has "railway style" automated barriers to check boarding passes and I seem to remember that these disappeared from Manchester.