My girlfriend is leaving the UK from Heathrow tomorrow. Is it possible for me to go through security with her so that we can be together until she goes to her departure gate?
There are some legitimate exceptions and general sneakiness but normally no.
You see an answer on a different forum on the same question.
There are also Gate Passes that can be issued by the airlines in some cases.
More then likely you don't qualify for either.
More links specifically on Heathrow
As far as I know, it is not possible in the United Kingdom. If anything, even if you could escort her through security screening, you would not be permitted beyond passport control without a boarding pass for a departing flight.
A potential workaround is to purchase a refundable ticket for a flight departing later from the same terminal. You would be sent through screening just as any other departing passenger, and then after seeing her off you could exit and get your ticket refunded (being mindful of the terms of the ticket). Personally, I think this is an ethical grey area, since you are tying up airport and airline resources without intending to fly, but it would be entirely legal.
Different countries have different policies. I believe Australia does not restrict airside access to ticketed passengers at all. In the U.S., one can obtain a "gate pass" for non-flyers from the airline, which is not unlike a "platform ticket" in British railway stations. Gate passes are usually restricted to people like parents/guardians of minors, interpreters, or companions offering assistance to people with disabilities or other special needs.