I think that this predates the redoubtable Mr Cox's company - although not him personally! I believe that he worked initially for the Marconi Company and then moved to ATV. I'm pretty sure it's not from the Marconi stable, but could have come from ATV. However, the BBC seems a more likely source of the idea.
The earliest example of PLUGE (Picture Line Up Generating Equipment) in my collection is also one of the items that I've owned for the longest time - about 1974 when I was a young teenager!
The item in question is a BBC Designs unit from around 1964 built in the familiar 'white module' style. It's a GE504/508/209 and the details of it are here:
www.bbceng.info/ti/eqpt/GE4_508.pdf
Also attached is a picture of my unit which has (sort of) survived the years. Back in the day, I soon had it working but it would take a little longer now. It came from M&B Radio in Leeds and cost me the best part of a month's pocket money.
One thing that it did teach me early on is the dire lack of safety standards in much of the BBC's early 'home brew' baseband TV kit with mains often treated like low voltage DC. Scary stuff - how did they get away with it, even then?
As to whether it was a BBC 'invention' would take more research, but this is the oldest example that I know of off the top of my head. Hope this helps.
Best regards,
Paul M