BA has twice now given me the weather middle finger due to "rain in London". One has to be an airline not see the comedy value in that one.
In LHR its all the capacity contrained situation at LHR, which causes delay, once a drop of rain starts to fall. However, no one forced BA to max out there slots. BA should just increase the block times to accomodate for that.
Thus, rain in London is not an extraordinary circumstance.