Wikipedia - never believe all you read. AGI - TBI - Abertis. From memory Luton is about 13-14 years into the concession agreement. Abertis can in theory walk away after 15 (of a 25 year term), or that is what was being proposed back in the days AGI took on the concession.
I must dust off an old PC with much more detail within. I will always remember Frankfurt airport walking away shortly after due diligence commenced, they were the firm favourites of the then, airport management.
The final three were National Express, Copenhagen airport and AGI (was Lockheed).