When I spoke to Britishfinescale about 5-6 months ago I was told that his intention was to go to the B6 range next (after the A5). I am guessing the timescale will be very much dependent on how much time manufacturing the existing range for sale takes up.
I will contact him again to see if he has an update on timescales. Once they are ready I will be stocking them
The OO and 2mm gauges were in fact his starting point and he expanded to EM