It was a diesel car. I know the link is the Daily Star but contains a direct quote from the Chief Fire Officer of Beds Fire Brigade.
https://www.dailystar.co.uk/news/latest-news/breaking-luton-airport-blaze-caused-31159205
There are charging stations but they are in Car Park 1, not 2.
London Luton offers six Tesla charger points, and four type 2 points that can be used by the public. These are located on the 2nd floor of Terminal Car Park 1...
https://www.bbc.co.uk/news/uk-england-beds-bucks-herts-67073446
All flights suspended, multi storey car park destroyed, up to 1200 cars burnt out, starting from one car fire.