Is a Wing Mirror Delete Legal in the UK?

Why?

The law requires every car to give the driver an adequate view to the rear. In practice that means an exterior mirror on the offside (the driver's side) and, in addition, either an interior mirror or a nearside exterior mirror, so the driver can see following and overtaking traffic.

A "mirror delete" that removes the door mirrors to clean up the bodywork takes away that required view. Even swapping to tiny show mirrors that do not give a usable rear image runs into the same problem: the test is whether the view is adequate, not whether something mirror-shaped is bolted on.

Mirrors are also a specific MOT item, so a missing or inadequate required mirror is a straight failure, on top of being a roadside offence. Small-arm aftermarket or towing mirrors are fine as long as the required mirrors still give a clear, adequate view; deleting them outright is not.