I'm afraid your recollection simply isn't very good on that one BP is scanned every time you go through security at LHR - whether o/d or connection. The scanning point can sometimes come quite a bit before you actually reach the security machines which may be why you don't remember it, but your BP was scanned.

As others mentioned, if you are on a single itinerary, you will be allowed to go through security if the airline considers that the arrival of your previous flight means that you have a chance of making your next flight. If they don't think you will, you will be offloaded and your BP won't allow you to go through security.

If you are on separate itineraries, however, you are effectively "starting" your trip at LHR and you thus need to comply with the airport security limits, which means "conformance" at T5 and (since 2013) "positive boarding" at T2, T3, and T4. There are only subtle differences between conformance and positive boarding, and for all practical purposes, in both cases, you must clear security at least 35 minutes before your flight departure time or your BP will not allow you to go through without seeing an airline agent - which can then ask to manually override the process if they believe that there is a case. Note that positive boarding is in fact a fairly adaptable/customisable tool, so for instance, if your flight is delayed, this will be reflected in the positive boarding deadline.

The reason why conformance and later positive boarding were adopted are not what is discussed upthread about not being able to exit the secure area of the airport. In fact, there is no real difference between LHR and most other European airports for international terminals on that front - departures and arrivals are segregated at most, so once you are airside, you can't just exit but instead, typically need to be escorted out. That is not LHR specific, and in any case, there is a procedure planned if people are "stuck" airside having missed their flight.

Rather, the reason why positive boarding was introduced was to reduce delays which were really bad at LHR at the time, and all the worse that the airport operates at (or some would argue beyond) capacity, meaning that a delayed flight will delay other flights and so on because there aren't enough slots, enough gates, or for that matter enough anything. So the point of positive boarding is that if you haven't showed up at security at T-35, then the airline is able to decide early on whether to offload you and your bag or not rather than needing to wait till gate closing time which is typically t-15 or t-20 for most airlines but can get even closer to actual departure if boarding drags on which is frequent, thereby sometimes virtually guaranteeing a late departure for the flight.

For this reason, airlines just love positive boarding (and conformance at T5). It gives their operational people useful data early, meaning they limit delays (which cost money), can manage any potential standby etc more effectively and so on. Not everyone likes the system (it is a relatively regular topic of discussion on the BA forum) but it's absolutely here to stay whether people like it or not.

Your perception is unfortunately not the perception of your contract If your BA flight is late, then BA is responsible for your CDG-LHR flight (ie for instance beyond a certain time, they will owe you refreshments and even compensation if the delay is their fault and goes beyond a certain time. However, they have no responsibility for your LHR-US flight on AA. That's a contract between you and AA and BA don't want to have anything to do with it. If AA want to consider this under their flat tyre rule good for them, but typically, most airlines (and indeed many insurance companies) specifically exclude arriving on another flight from those. Incidentally, note that all flat tyre rules are solely a facility granted by the departure airline to the passenger without involving in any way whatever company may or may not be responsible for the passenger making it late to the airport. As others mentioned, many airlines have such rules meaning that if - e.g. the tube breaks down and as a result you arrive too late at the airport, the BA or AA of this world will typically move you to the next flight at no cost as a result. It is not TfL which is doing anything for you, nor would AA or BA try to get TfL to pay for the cost of moving you to another flight. So in effect, when you book separate tickets, you just need to factor in all those risks because you "own" them entirely and will be held responsible for them.

Incidentally, no, you don't need to book "nonstop", you just need to book "on one ticket" which is not the same thing, and your buying separate tickets through ZRH, CDG, FRA, AMS or FCO will make no difference - in each case, separate tickets will mean your risk if you miss the flight. So sure, you were surprised that things turned "red" at security, but to be honest, that's a bit of a red herring - positive boarding is pretty aptly calculated, and if you were allowed to scan your BP at T3 less than 35 minutes before flight departure, between the time it takes to clear security and the distance to some of the gates, in most cases, you would reach your gate after the boarding deadline time any way. In that sense, the main difference between LHR and ZRH is not so much that ZRH has no positive boarding, but that it is a lot smaller. It is worth remembering, however, that at ZRH, in addition to security screening, you'll need to factor in emigration clearance which can take a little while in high season.

Either way, I suspect that the vast majority of the people who have answered in this thread would simply advise that you don't book two separate tickets with merely 2 or 3 hours between them - be it with a transfer at LHR, ZRH or any other European airport - especially when the second flight is your "expensive" tatl J flight which would cost you a huge amount of money to rebook if you miss it and are considered a no show. This simply isn't safe, regardless of positive boarding.