Tbh I'm not sure about Cornwall, but there are definitely parts of the South Coast which are very, very cheap (and shitty, which is why they're cheap). Brighton's obviously cray-cray expensive but places like Camber in East Sussex you can get 2 bedroom flats for under £90k. Obviously the jobs market round there is crappy (especially in Winter) but you can get to London in about an hour from the closest station (which is admittedly a few miles away).
Edit: I remember driving down to Cornwall once and found it bizarre that the "main route" from London to Newquay got gradually worse as you got further and further away, with a pretty steep drop past Bristol. At some points it was like a small country road, and this was THE MAIN way there. Savages.