Doesn’t appear that GWR▸ ran any extra services between London and Cardiff yesterday. I know the long term plan is to use 387s when there are events at the Principality Stadium. Sure would have been useful yesterday.
There was definitely at least one extra - I think it departed Paddington at 09.18. I nearly jumped on it to go to Bristol Parkway before I noticed it was advertised as calling at Cardiff, Bridgend and Port Talbot Parkway only. It was a 9 car.
A quick check on realtimetrains shows there was another extra at 10.18. I don’t know what time kickoff was but I would guess GWR expected most passengers to travel earlier so short forming the 12.48 was the least worst option.