Yesterday there were a large number of cancellations and delays at Gatwick Airport due to a shortage of air traffic controllers.
Other sources detail how flights were diverted to airports as far away as Cardiff and Bournemouth- quiet airports which had capacity.
Bournemouth is a regional airport which mainly handles charters and scheduled flights to the Channel Isles (Jersey, Guernsey and Alderney), but has a long runway and lots of capacity so does get used in such situations. The airlines would have laid on buses back to London, but that takes time for them to arrange and then your time back on the land transfer.
These things happen at short notice, randomly, but illustrates again why you should build in buffer time in London, rather than looking to move straight through.