I'm not 100% convinced it's Puch, unless they used two slight different variants of their tube crimping & welding technique at their factory in Graz. I have a Puch 3 speed which, IIRC I got in 1984/85, and whilst the tubes are crimped & welded, the join and dropout has a heavily embossed criss-cross pattern on it.
The OP's frame actually looks more similar to the construction of my 1985 "Astra" roadster, nominally made at the Elswick-Falcon facility at Barton-on-Humber. The tube crimping & joining and rear brake plate are virtually identical, although the lugwork is different. I suspect my roadster frame is a rebadged budget import and not a proper UK build.
I was saying that it has certain similarities in build as a Puch. I agree that the rear dropout is different in the way that they are welded in . The Puch does have marks left in it from the welding process, a bit like a Blacksmith's weld. Top of the range Puch cycles were made from Reynolds 531 tubing and were made in the conventional style .
It is a puzzle , having a square badge should help identify it as not many use that shape .
My Raleigh Pioneer 18-23 Hi-tensile frame from '91 has a flat brake plate, as did the '92 ladies Pioneer I used as a mechanical donor. My Reynolds 501 version has a tubular brake bridge, so the flat plate looks to be a budget frame only feature.