I haven’t tried with the mapping ordnance maps yet no. But what I have managed to do is use QGIS to take map image file and manually geo-referenced them, then export as a geo-referenced PDF file that loads in the free version of Avenza Maps. My main issue when I last used it was I couldn’t import my own GPX files on top of it, but I think they added that in the last Avenza update.
I used this system with my digital version of the Slingsby Cederberg maps. I tediously merged all the separate map files into one complete map in photoshop and then mechanically geo-referenced the single map in QGIS. I would guess it is not perfectly accurate but when I was last in the Cederberg it worked well. Was very very happy to have a map with so much detail straight on my phone with my live location on it. Have suggested to them that they look at making their maps available like that by default.
So my guess is that you could do with this the mapping ordinance files. In fact I would assume with a lot more ease and accuracy. I haven’t gotten maps from them since my student GIS days, they might actually already be in geo-PDF which Avenza will read.
Was half a year ago when I set this all up. I know I ran into an issue with Locus Maps doing this, but for the life of me I can’t remember what. Just hazy images of trawling through their forums for an answer 