Royal Mail have started including barcodes on standard Air Mail services. These barcodes can be seen on your Shiptheory shipping labels that uses Air Mail services, and is used as references on the border during shipping. These aren't tracking numbers.
In here, the barcode on the right is the new reference code we are referring to in addition to the usual 2D barcode on the left side. UG014347824GB is an example of the new reference that is being implemented now on Air Mail services.
If you have any more concerns about this, please feel free to contact Support.