Hi, only C-roads and U-roads in the UK should really use the tag highway_authority_ref. Regardless of what the sign says, the ref as stated in the reference I tagged on the ways is A8(M). Signage was probably only changed for simplicity, however the ref hasn't. There's likely lots of examples of this, whether intentional or whether down to botched/incorrect signage. As for the tagging, I feel highway=motorway is more appropriate, as the A8(M) has it's own number outright.