Sam, to start, the seat lug looks like a standard cheap pressing in the pre-war style with ears for the seat stays attachment, which have solid top eyes, also used post WW2 as frames had to be made out of any stock bits.
But I have never seen a seat tube not cut flush with the top of the lug. ( at this sort of age)
If you have 120mm rear end this would surely have been 4.5 inches, is there any sign (slight curve) when you look down the seat stays.
The bottom bracket looks like a casting, but not BSA.
Could it have been built with one Chater fork end and one other?