Here are today's (Monday 29 May) short notice cancellations. No longhauls it seems. Their return to LHR services will also be cancelled. These are all either equipment or crew shortage, and would be eligible for EC261. However they do have alternative services close by so it will depend on timings.
BA944 DUS
BA1452 EDI
BA1412 BHD
BA712 ZRH
There are also some oddities with HAJ and MUC. They seem to be operating, indeed the morning HAJ was airborne 5 minutes after departure time and will arrive nearly half an hour early into Hannover; but they were briefly cancelled.