Which? report
It's hardly surprising that the top and bottom three are those listed. The top three are lightly used, and in the case of SEN and DSA, modern facilities.
The bottom three are overcrowded and not user-friendly. All three are on small, limited sites. They have all suffered a severe lack of investment over the years.
In Luton's case, that is due to its unusual ownership model which caused lack of proper investment by the Council during its years of operational ownership, followed by restrictive bouts of short-term investment since. Because of the franchise model, operators will only invest at the beginning of the franchise period for sound commercial reasons. The dead hand of council ownership prevents the operator from planning any extension outside the airport boundary, despite the wishes of the market.