These BMI charters have been going on for years, winter and summer. They use marginal time in the aircraft programmes on Saturday afternoon and Sunday morning (and a few overnights) when BD has more LHR slots and aircraft available than it can use for its normal destinations. The aircraft are the regular scheduled fleet rather than the charter-config ones BD use more permanently out of some provincial points. For many years some of the Heathrow flights were to oddball places on the north coast of Spain.
Strangely I've never seen any publicity by a travel company for any of these flights; their marketing must be very low-key.