Having recently tried my first entry to what I thought was an amateur exhibition for Amnesty (art generally, including a minority of photography) but found it mainly professional bin-emptying, I can tell you both what I did do (on Andy's signature theme) and what I should have done.

Correct orthodox & possibly what I will do for next local club comp:

mount without further bordering or even necessarily chamfer in pale cream mount around 50mm over size for A3ish, slightly bigger margin at foot. 40cmx50cm still current at local club, which means A3 really maximum.

What I did and will continue to do for home and friends:

Mounted onto 16mm board with the edge of the photograph folded down over the edges of the mounting board. A3+ - looks stunning on white background from 40ft away

The latter is not practical nor probably within rules for most places, but am convinced it is good for photos to be photos, not pretending or aspiring to be substitute paintings. Will be working on more practical compromise, but thought you might be interested in a clarification of what I think the issue is.

For selling, cute trick is to provide mounted as above in cellulose wrap in rummage canvas-on-folding-frame for folks to frame themselves

PS hope you will be coming to proposed Marbury meet on 14th June and perhaps we can bring a few examples?