I've never looked at those tables before but I'd advise caution looking at one set of numbers. For example 2012 Jet2 Complaints/million 42.1 vs Total airlines 32.9 ; 2014 to Q3 Jet2 108.4 total airlines 104.2.
Isn't it in the nature of airline operations that a few events will give you a spike in the data.You need a long run of data to be saying that A is significantly better than B or has been getting better/worse over time.
I wonder if foreign passengers on the big scheduled airlines are a bit less likely to complain to the UK CAA.