To my awareness no bofors 20/25mm guns were used for home defence in the UK. In fact I have never even seen a reference to them anywhere in a ww2 context, and indeed on reading that page it would appear that they are both post WW2 models (apart from the m/40 which I don't think the UK purchased).
The most likely explanation is Bren or Lewis. I've made it clear I'm only floating a theory to be explored.
I don't know whether any 20mm m/40's were purchased or used by the UK either. My only point in relation to either this or the 25mm m/32 (which was in service with the Swedish Navy in 1936 ie not a post WW2 model) was that they both existed during WW2.
Jonney has said that "they did seem a lot bigger than your average type 22" Because of this and the other anomalies, I'm keeping an open mind.