Wellll, as somebody involved with the handling of VA baggage at Heathrow, I feel that I should comment ....
How it happens at Heathrow ...
It doesn't matter what label *you* put on your bag, it is the label attached at check-in which is read by the machines (laser scanner on the bar-code) which decides where your bag goes. If you're in UC then it goes into the 'J' bin (as we know it), and if you're in UC it goes into the 'W' bin. Almost all the other bags go into the 'Y' bins. A selection of bags (usually the late arrivals, or odd sizes and shapes - ski's, golfclubs, surf boards, etc) are placed into a separate hold at the back of the aircraft.
The baggage bins are delivered to the back of the aircraft, and are loaded by us! The 'W' and 'J' bins, and the one(s) containing Transfer bags (ie, bags for passengers flying onwards from your destination) are the LAST to be loaded. In an ideal situation the 'J' bin should be in the doorway of the hold, but it could be blocked-in by bin(s) of Transfer bags.
When the flight gets to the other end the 'J' bin *should* be first off the aircraft, or firstly the Transfers and then the 'J'. The 'Js' should be first on the baggage belt at the other end (in theory!).
For an inbound (to Heathrow) flight, we are all waiting for the flight to arrive. There are people who's job is just Transfers and those who's job is just 'Js'. They are the first to collect their baggage bins from the aircraft; they go to different places in the baggage hall, so we can just be concerned about the 'J', 'W', 'Y' and loose baggage. The 'J' bins have priority, and we have to deliver them to the baggage belt first. It does happen that the 'J' bins are delayed from the aircraft, so those of us doing 'W' and 'Y' have to wait. On a few occasions I have got the the belt first, but had to move aside to allow the 'J' bags and 'W' bags to be done first.
It does happen that some 'J' bags are loaded loose into the rear hold, and they *should* be unloaded first. But when you open the door and are met by a wall of 50+ cases (and cargo, and courier, and push-chairs, and child-seats, and ski's, and surf boards - you get the idea!) it is not easy to find one or two 'J' bags in the pile. These bags are manually unloaded, manually loaded into a cart, and then manually unloaded onto the baggage belt. Please spare a thought for our backs! Unfortunately (but not always) these loose bags are usually the last things to be put onto the baggage belts.
Regarding 'multiple belts' or 'switching belts' - I have never ever seen multiple belts in use at Heathrow, and the only time that I've known a belt switch to happen was when somebody (okay, okay, it was me!) put the VS010 bags on the VS020 belt; but that was before the belts had started and any of the passengers were in the baggage hall. We just changed the screens in the baggage hall! Oops.
(Hopefully I won't get into trouble over saying all this!)