Quick update: landed at 23.25, baggage arrived well past midnight. Booked a cab via the hotel reservation desk at arrival (79 quid) and got to Southwark safely around 1.15am.

Called Customer Relations this morning, and had the nicest, most accommodating conversation with an agent who apologised profusely for the delay, approved the decision to take a taxi rather than opt for the night bus, and arranged on the spot a bank transfer for the full amount paid for the taxi. When I asked for the address for sending the receipt she said "don't worry about that, no need". Very very well done, BA. Very well done.