Sure someone will beat me to this but... Thomson and First Choice Airways used to be two separate airlines, but merged their operations a few years back and now operate under the "Thomson" name and callsign. The aircraft still in the old First Choice livery are leaving the fleet before too long, so were not re-painted.
On the subject of "Castrol" - are you sure you're not confusing Thomson and Thomas Cook Airlines? Thomas Cook use the callsign "Kestrel".