[oscillon]

From what I recall, there are three pitches of LEDs used in B - 1.5mm is used for the towers, catwalk screen and touchscreen; 2.5mm is used for the main section backdrop videowalls and 4mm is used for the upper 'floor' backdrop videowalls.

I suspect that the regions are a mix of 1.5 and 2.5mm - mirroring the national set-up.

Indeed, from the article below, the national set has 1.5mm pitch for the main presenting screens - i.e. the towers, the catwalk screen and the weather/touchscreen.
www.tvbeurope.com 
NewscastStudio got it wrong this time.

So the key question is - what is the pitch for those three physical walls (1.5 or 2.5 mm) and whether moiré effect in smaller sets can be helped by choosing smaller-pitched (but may be smaller-sized, due to the set limitations) walls?

Northern Ireland has three LED walls the same as an English region, but has a separate green screen AR section strapped on the side - as seen in the screenshot below.

However, as their studio is quite large, essentially the cameras can be turned almost 180 degrees in the opposite direction to the physical set. This is where the green screen is located. So essentially, you could say NI almost has a full box (i.e. three sides of LED screen and the last side is CSO). I suppose it was a cost effective way of extending what they have without splashing out on more expensive LED panelling. 

Ah, thank you both so much for clarifying, this is where the initial confusion arose from. So it is the FOURTH wall that is virtual and criticised, leaving only one small angle of the studio not on display..