That £300 return subsidizes train services in rural parts of cornwall. That's a political decision. Other options are to have more taxpayer subsidy, higher prices in Cornwall, close services in cornwall, or reduce operating costs (which are heavily staff and rolling stock based -- something that HS2 will slash)

P.S. I just looked up an equivalent London-New York economy flight (flexible flight leaving this evening and returning tomorrow), and it's £2,481.02