This is of cause where the 504 is famous - Africa. Peugeot had great success rallying there. The cars are relatively simple, but extremely strong. I had a 504 diesel for a while and the thing was unstoppable. It was a superb tow car, and would do 90mph (140kph) once I fitted it with a 505 5 speed gearbox.
They're still manufactured and marketed by Peugeot, the 504 is listed as a current model on the Peugeot web site:-
Peugeot 504 saloon (fr: berline)
Peugeot 504 estate (fr: saloon)
Four wheel drive 504 Dangel
Dangel still convert Peugeots to four wheel drive, see their web site.
I wonder what it would cost to buy a brand new one in Africa and bring it to England. Northern Africa is a possibility however; from Algeria to Europe is a quick hop across the water to Spain. Current prices are available on line in Argentina from www.maldonado.com.ar/maldo7.htm. The Argentinian model looks a little different, see www.peugeot-argentina.com/Peugeot/P504.htm for a picture. If the car were rust treated from new then I'm sure it would still be going in 25 or 30 years time. Would be a much more interesting new car than a Mondeo.