No, not really “sick” as such.
All aircraft have a series of checks (sort of like car routine servicing) performed at regular intervals, “transit” between every flight, “daily” …every day, “weekly” etc…all/most of which can be done either with the thing parked outside the terminal (normal for the transits and daily’s) or maybe when it’s parked up at the airline’s maintenance facility at the airport. At BA that area is over near Hatton Cross Tube station to the east side of the airport.
However some of the really detailed deep checks, known as “majors” get done very few years and take weeks to perform. Those involve almost stripping the airframe back to bare metal before checking and then rebuilding the thing, fitting new interiors etc if applicable.
Rather than tie up concrete and hangar space at Heathrow for long periods BA decided quite a few years back now that it was more economical to do major servicing away from London and so set up the facility at Cardiff (I think these days some majors on some types get done by third party contractors out in the Far East).
I’ve been inside the Cardiff set up a few times and it’s a very very impressive engineering set up.
On the flying side and with reference to the OP, and last night’s flight: pilots at BA occasionally get tasked with flying an aircraft from LHR-Cardiff or vice versa, with a taxi ride in the other direction. If they are lucky they get a real treat and fly both ways, taking one airframe out to Cardiff for it’s major and bringing a freshly serviced one back to London…
BTW the slightly odd callsign/flight number for a BA flight is explained by the fact the aircraft is on a non-revenue positioning sector.
Hope that’s of interest.