To be pedantic (pedantry on FlyerTalk, never!), the exit controls from the UK (re)introduced last year require the relevant transport operator - in this case Eurostar - to do the checking and scanning of passports and then pass the details on to the UK government (specifically the Home Office, of which the Border Force is part). So it's not personnel from Border Force who actually do the exit check and passport scan.

(I'd be curious to know how quickly this information is transmitted to the Home Office, and if it raised a flag how quickly the authorities could then act and get the Border Force or police to speak to/detain said person. But Carlos - this is totally irrelevant to you, you're fine!)