hello Nicholas, welcome to the GMB

........ firstly, mirrors aren't my thing, but thought it worth commenting............
Your conclusions might be suggesting there is the possibility that this example is perhaps of modern construction, rather than period - it's always worth remembering that art deco is the name given to a style, and not necessarily to pieces which are from a specific period - c. 1920's to c. 1940's, and due to its popularity there is much contemporary decorative 'art' made in this style..... in fact I don't think the style ever goes out of fashion.
regret I've nothing really useful to add, but would suggest that if this mirror were seventy years plus in age, then evidence of this should be apparent.
Between the wars, mirrors for the domestic market were made in truly vast numbers - virtually every home had them, and the oval shape was perhaps most common .... in the hall, the front room, the dining room ............ I'm looking at one now in the room where I'm sitting, although it has a wooden frame.
What is the condition of the chains, the extremities of the glass, is there any loss of silvering etc. .... is the condition too good to be true?
Where did you acquire the mirror? What makes you suggest English - the style was widespread throughout Europe during the period of which we're speaking, and then there is the possibility that yours is a modern copy of an earllier period design.
Yours may well be pre 1940, but being the pessimist that I am, and bearing in mind the vast profusion of these things, my thoughts are that unless you are exceptionally lucky then this is going to be a non-starter in terms of finding a maker, country or manufacturing year.
However, hope someone can prove me wrong, and best of luck to you in your search.