Your Milner B561 safe looks rather familiar to me - very similar to the ‘Merlin’ quality of the 1960’s.
It came in 3 sizes, A,B and C. Does it measure 28″ in height by any chance? That would make it a size B.
There’s something else about it which makes me think ‘Whitfield’. This was an earlier Milner post-war safe which had a handle virtually identical to the one fitted on your safe.
The plate or badge is not one which I recognise which in turn makes me think export, and which could account for the mixture of handle and plate, either/neither Merlin or Whitfield.
Anything on the back of the door? Are the bolts fastening the door rectangular or round?
I have a copy of the original price list if it’s a Merlin.