Airport inbound flow regs are nomally based on weather, capacity, sickness or "other". If the predicted landing rate for LHR is 40/60 or less, due to a combination of the surface wind and the 3A wind,this is normally attributed to weather (CFMU WA84), obviously if CB's/TSRA's are forecast this will also be factored in.
There are CFMU staffing and capacity codes that relate specifically to either sector/approach functions (SE/CE) or the airport of destination (SA/CA).
Sometimes the WA regs may appear obscure when the wx looks okay but it's probably down to the 3A wind reducing the landing rate.