I don't really agree with this 'declining grid' narrative the renewable lobby has pushed. Yes there is upgrades to be done, etc etc. But peak UK electricity demand is down from ~65GW to ~45GW (which may change, but doesn't look to be).
Nearly all of the cost on the grid is to do with renewables, not 'general upgrades'. We would not be building 10GW of HVDC from scotland to england. We wouldn't be doing a drastic 275kV -> 400kV rerating and duplication in the middle of nowhere scotland otherwise.