Bit of a random starting point for discussion.
Iberworld had an average delay of 34 min last year for their flights in and out of the UK. This is very similar to Air Europa, Futura and Spanair (32, 36, and 37 min, respectively).
Monarch, Air 2000, JMC, Britannia and Excel all did better than that. MyTravel had a bad year then but are now down to about 20 min apparently – thanks largely to the new team there me thinks.