I don't see any mention of a label in the Tester's manual
6.1.6 Coupling mechanisms and towing equipment
You must inspect all types of coupling mechanisms and towbars fitted to the rear of a vehicle, including fifth-wheel couplings.
You don't need to inspect emergency towing eyes.
You must remove tow ball covers to inspect the tow ball.
If coupling mechanisms are behind access panels in the bumper, bodywork or removable panel, you must remove them to inspect the coupling mechanism unless tools are needed to do this.
When checking coupling mechanisms, you may also need to check inside the luggage compartment and lift loose fitting mats or carpet.
You must test retractable towbars in their ‘in-use’ position. However, if you need tools to do this, you don't need to do it.
If a tow ball or pin isn't fitted at the time of test – because it's detachable, it's been unbolted or otherwise removed – but the attachment brackets are still in place, the brackets should still be assessed unless they have been deliberately rendered unfit for further use.
There might be movement (‘play’) in some detachable tow balls between the receiver socket and the tapered swan neck fitting, with up to 3mm movement measured at the ball end.
You must reject the following:
pins, jaws or hooks that've worn by more than 25% of their original thickness
pin locating holes that've been worn or elongated by more than 25% of their original diameter
tow balls that are obviously excessively worn
Many ‘bolt-on’ type tow balls have accessory devices between the tow ball and its mounting flange. You should only reject these if their fitment is clearly likely to adversely affect the roadworthiness of the vehicle and its trailer.
https://www.mot-testing.service.gov.uk/documents/manuals/class3457/Section-6-Body-Structure-and-Attachments.html#section_6.1.6