I can confirm BT had many hundreds, maybe thousands of 13A sockets restricted to 3A or 5A [it is just possible I may have some with their sticky lables] but this was for a completely different reason. The exchanges originally had 5A cast iron sockets mounted on the older equipment racks but newer installations had 13A sockets, in order to avoid incompatibility they went through a program of changing the 5A sockets to 13A but they left the original 3/0.029 wiring and 5A fuse. The earlier lables showed 5A restriction but it creased a problem with 2 or more 'high power' devices in use so the later lables showed 3A restriction. That was nothing to do with socket or plug current carrying capabilities, just the wiring and as we are so often quoted 'the fuse is only to protect the cable'.