I count 306 seats where Labour are 1st and the Conservatives 2nd, or Conservatives 1st and Labour 2nd.
In the other 326 seats, either the Lib Dems, Reform, Greens, SNP, Plaid Cymru or independents are a top two party. Where most voters live, the traditional Labour vs Conservative debate is no longer the relevant one.
Constituencies don’t vary in population nearly as much as your example. You could do more accurate calculations using This data but I doubt it would make much difference. And yes, fuck FPTP