The seat tube measurement is significant. By the end of the 1970s there were very few models made by Falcon in 531 butted tubing. In the 1976 catalogue (just a couple of years before your frame seems to date from) only three models have 531 butted:
model 76 San Remo Equipe (which has the distinctive seat cluster),
model 84 Cote d'Azur
model 88 Super Tourist de luxe (chrome ends to the forks).
The model 84 Cote d'Azur had been made on and off since 1963 and the model number continued in 1983, renamed the Alpine.
For my money, the lug type, the detail at the top of the seat stays, and the rear brake bridge detail put it in the 1978/79 ish era. Falcons are notorious for finishing their models slightly differently (I have photos of between 400 and 500 Falcons between the 1960s and the early 1990s and there are all sorts of frame decoration anomalies in there, so yours would not be unusual to have the white panels on seat and bottom tube.
That is about as far as I can get! Without doubt it is a nice good frame which deserves a top quality rebuild and repaint. Do go for stove enamelling if you can.