OK, I tried to explain it in a few pithy sentences, and couldn't, so I did what I do when I'm stuck for a pithy sentence and went to Wiki to see what they could do, and here it is. It speaks for itself.
Part of the problem is the large number of types, which is a BA inefficiency which causes maintenance inefficiency. But who in their right minds would sit down and, with a clean sheet of paper, design a maintenance structure like this?
Please note that I do NOT mean that the various maintenance sites are not themselves efficient, top quality etc. It's the whole creaking structure that's so wasteful.
British Airways Engineering is the aircraft maintenance subsidiary of British Airways plc which provides support services to British Airways and other airlines. It is responsible for the entire BA main-line fleet maintenance, cabin interior conversions and general ramp maintenance work for both their own fleet and other airlines. It also sends some of its own heavy maintenance work out to other companies, although the vast majority of aircraft and cabin interior work is still carried out by BA Engineering itself.
British Airways Engineering was formed from the merger of the engineering divisions of BOAC and BEA's when the two airlines merged in 1974 to form British Airways. Today, British Airways Engineering has hangars at Heathrow (its base), Gatwick, Glasgow and Cardiff International airports as well as hundreds of line maintenance stations (engineers based around the world to do minor 'line' maintenance such as routine daily inspections or minor repairs). A team of about 30 engineers are still based at MAN earning revenue from a large number of third party airlines. It also consists of the Avionic division which is based at Pontyclun, South Wales, which services the majority of the removable avionic units, and some mechanical components.
There are now 3 main maintenance hangars at Heathrow, these being (TB standing for technical block) TBJ, TBK and TBA along with a huge maintenance hangar called TBE and engineering's main office building called TBC.
TBJ & TBK (4 bays combined together): All of BA's 747 fleet 'light' maintenance is done here. The hangars are complete with component and engine maintenance facilities, workshops and a small interior 'patching up' line (to do minor maintenance on seats). Outside the hangar there is a parking area for the up to 8 planes and an engine test pit.
TBA (8 bay hangar): Airbus A319, A320, A321, 757 and 767 'light' maintenance along with almost all 757 and 767 fleet 'heavy' maintenance is done here.
TBE: The minor or 'casualty' hangar, is where most minor work is done, engine and leg changes for example and any work that can be done in the short period of time an aircraft has on the ground. The section covers all the airline's Airbus fleet along with the Boeing 757 and 767 aircraft. Engineers based here support the line maintenance and major maintenance areas as required and the section is now known as the FSU (Fleet Support Unit).
TBC: Engineering's main offices and some training facilities are located here, and is where most support engineers and senior managers are based. Planning, development, finance and technical support sections are all based on the 9th floor. The ground floor houses the engine workshops.
British Airways Maintenance Cardiff (BAMC) is based at Cardiff International Airport. Here, all 747 and 777 fleet 'heavy' maintenance along with most of the airline's long-haul fleet interior conversions is done. British Airways Avionic Engineering (BAAE) is also based in South Wales which is responsible for the repair, modification and overhaul of the vast majority of the avionic and electrical components used in the BA fleet and some of those used by other airlines. It is situated in three purpose built workshop buildings at Llantrisant.
British Airways Maintenance Glasgow (BAMG) is where all Airbus A319, A320, A321 and Boeing 737 'heavy' maintenance is done. The hangar has avionic, composite and an interiors workshop. All components that are not overhauled at Glasgow are sent to London for overhaul. Line maintenance at Glasgow is carried out by hangar engineers, as well as a dedicated shift for night stop-overs. BA's B737 'light' maintenance is carried out at Gatwick.
Get the point? It might look OK to engineers, but it must rank as the most wasteful arrangement that could possibly be devised, in terms of resources consumed for the result.
PS....There's a clue to how this all came about in the words
merger of the engineering divisions of BOAC and BEA's, and in the fact that there are large bases in Scotland and Wales, which has as much to do with politics as common sense.