And indeed this is the current position, on the lower level of the main departure hall, the nearest gate being A15.
That facility is very useful, since you can also use it for things like Tracked 24, which gives essentially the features of recorded delivery, now that signatures on delivery aren't being collected. This is how I use Randox tests rather than their drop boxes, however the mouth of the boxes in Heathrow is only about 1 cm deep, so it's more for cards and letters rather than PCR kit boxes. Payment can be made online via cards, Paypal and Google Pay, you can pay for and then print your item up to 1 week in advance of putting it in the box.