Confusing story for two reasons - iPhones and ipads in my experience, every time, are returned with UPS and have been for years due to their value and the contracts Apple has with them in the UK. Also, Apple use logistics companies to deal with all returns so I have never seen the word "Apple" on a returns label.
Now let's say you did bizarrely were provided with a returns shipping label/box for Royal Mail, that would never insure the value of the goods, Apple has made the mistake. Then once you handed over the package to Royal Mail there is nothing else you can do. The contract for shipping is between Apple and Royal Mail because Apple provided the returns label. But let this be a warning to all in the UK, do not send any items that cost hundreds of pounds with Royal Mail without using Special Delivery with additional insurance cover to the value of the goods.
I had this issue with an iPhone 12 return also before Christmas. UPS "lost" my return, this is the second time this has happened for me and I believe strongly that somewhere in the system items are being blatantly stolen. Apple may not put their name in the returns address but their brown box packaging is obvious and anyone who works in deliveries will know the big companies packages.
It takes Apple and UPS a month to investigate, and I had to constantly chase Apple who wasn't that helpful but always used language that told me my refund was safe because UPS are Apples service partner and it is a contract between them - also my return tracking showed "Collected" status but no more. I did get my refund but it was frustrating. If the pickup courier had not scanned the return package on my doorstep (and the system acknowledging it) I would have no proof of return as they do not give receipts at pickup or at returns desks (saving paper).
This has knocked my confidence in purchasing anything from Apple moving forward. UPS have always been one of the more reliable couriers in the UK at least, I would put DPD on the same list, but having had two returns go missing in the last year I would never want to use Apples postal returns again.
I am prepared to believe that during the stress to the system covid has created for these courier companies that there are unscrupulous staff taking advantage and stealing packages with little fear of being caught because of the chaos. This happens at Christmas as well because of the volume of packages. Otherwise, there must be a really dark corner in the sorting warehouses up and down the land where all the "lost" packages go and no-one looks, along with all the worlds missing socks!
Years ago I returned a faulty Google Nexus phone and was provided with a Parcel Force returns label. This package went "missing" as the tracking did not update - tackling the issue with the Google store was going to be a huge challenge. Luckily I managed to get the phone number for the collection depot and spoke to a member of staff who gave a damn. They raised it with their shift manager who investigated and found the package under the collection drivers van seat. I was told that no packages should be in the driver's cab so draw what conclusions you want from that.......