I will say there is some truth in this, if you look at EIR on a lot of routes their starting fares
are £70-£80 return. Indeed FR are very competitive - at times. Dont however expect £20 return to last forever. There is no profit in that whatsoever. You mention BHX, I will give you an example, since some point last year FR and EI have been engaged in a fares war and even EI have £19 one ways in place this long time. However for a period of years even prior FR had been cutting capacity and at times starting fares were £70 return and often EI were undercutting them and had become dominant.
BHX-NOC seems to work best with a low cost carrier, MYTlite and WW both had their success in terms of pax numbers and I wouldn't doubt FR would.