Airports with modern A-SMGCS and selectable taxiway lights are not limited by taxiway guidance in low visibility, but protecting the LLZ for the ILS.
Yes, down the road at some point we'll have GBAS CATIII, work is already being done on reducing the LSA for certain aircraft types. Lessons learnt from the increased LSA required if an A380 is taxying next to the runway are being applied to other aircraft.
That still doesn't mean that an airport in the Thames Estuary would be free of the capacity issues every single other large airport has in fog.