My KH front brake plate moves exactly as you describe when the brake is applied but by about 0.1mm, according to my rough measurements with the vernier calipers. That brake does work, but is not the best of the type I have had. Equally, it is not the worst. It is a sod to set up well and goes out of adjustment in under 1000 miles but it is not bad enough to have made it to the top of the list of priorities for attention.
I replaced the dust cover with a stainless one which I attached with 4BA allen bolts, as is my habit. I have since wondered whether the original spot welds added to the rigidity of what is one of the flimsiest brake plates Ariel made.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.