it is likely that this is a proactive cancellation based in ATC reducing the landing slots at LHR because of weather. In which case EU261 does not apply.
Just because other airlines are operating flights from both airports is not really relevant. If BA (and other LHR airlines) have been told to cancel X number of flights then it makes a choice which to cancel but that does not mean EU261 applies.
There is (or at least used to be) a list of such proactive cancellations on the CAA website