Sean Dell, you asked whether LVPs were an RVR based phenomena. LVPs are applied when RVRs are 600m or less or cloud ceiling is less than 200'.
No matter what the forecast might be, if the met observations fall within these limits the procedures must be applied.
As for this not being fed back up the line to become a slot delay at point of departure, well it is. Obviously not as efficiently as you would like but weather forecasting is not an exact science (nor is flow control).
It would be very embarrasing for ATC to apply excessive flow controls only to see the fog/cloud lift earlier than expected, the landing rate increase and the stacks empty before delayed flights can leave their point of departure.
On the other hand, if ATC cancelled flow restrictions on the basis of a forecast weather improvement which did not materialise we'd be left with 30min EATs plus an over delivery of maybe 15-20 aircraft in the next hour (and nowhere to put them).
WHBM, you make a very good point but inevitably some days the capacity at Heathrow will be reduced.
This may be down to low visibility, snow, strong winds, aircraft emergencies or airfield unservicabilities. If these conditions reduce capacity by 20% some days would you propose the airport runs at 80% capacity every day to obviate domestic cancellations ?