This has been discussed a number of times, and the simple truth is that the orange/yellow priority tags are simply visual clues.

The information as to where your bag is a priority, or not, is in the bag's file, which is read/accessed through the barcode printed on the bagtag.

On short-haul, there is no segregation of bags according to whether they are priority or not. The only segregation done into LHR is between transfers and direct. This is because there's usually very few bags that are premium and London-terminating. Adding an extra bin for those could cause other bags not to be loaded (if the flight is otherwise full of bins) or in any case increase the fuel consumption of the flight, for the net weight of an AKH bin is around 70kg.

On long-haul, flights have the following segregation:

• short-connecting bags
• long connecting bags
• First bags (when available) London terminating
• Premium bags (J-class) London terminating
• Economy London terminating

*I don't remember where Gold/Silvers not travelling in premium cabins go. I think they go in the 'economy' bin, for otherwise often you end up with more premium bins than not.
The building of bins in the airport of departure is automated. In other words, the workers will only load short connecting bags onto the short connecting bins. If they mix bags, when they scan them the 'gun' will send an error message. In more refined airports, only the appropriate bags will be released by the system for the bin in question.

As a rule, short transfers/transfers overall on shorthaul are by the door and out of the plane first for they are the most important. Then the offload is First, Premium, Economy, Long connecting. This is usually scrupulously adhered to.

When things break down a bit is the process of offloading. Especially at LHR. The driver with the short transfers will be the first one to peel off from the aircraft side and will drive his tug and bins (tug-and-three) to the nearest input point, where the bags will be fed back in the system. The second driver will normally get the First bin, plus one or two more bins of premium bags. NB: a bin carries around 40 bags; on flights like a very busy LOS when it was on the 747 Super Hi-J you're likely to see 4-5 premium bins. So two trains will be required.

Once the premium bag driver has arrived at the arrival belt for the flight, he should start offloading the F class bin first. All he has to do is to step out of the tug, read the A4 card on the side of the bin that says "First" and start from that one. Failing that, the visual clues are the yellow tags. More often than not they don't do that, especially at LHR, and they offload whatever is closest to the tug, which could be a J-class bin, or even a 'normal' bin if the offloading was done badly or the loading was done even worse.

That's it in a nutshell.