A pair of reserve Lions could extend British battleship service

Would a modernized Hood and completed Lions make Vanguard unnecessary?

Potentially; retaining two Lions could also let one supply parts for the other and extend a battleship's postwar life.

A modernized Hood and completed Lion-class battleships could remove the need to build Vanguard. Two Lions might then remain after the war while the KGVs and older battleships leave service. Keeping a pair in reserve would allow one to provide parts for the other. That creates a speculative route to one ship surviving as far as the Falklands War era. It depends on the later decision to retain the ships.