Surely another factor that affects the UK perhaps more than some other European countries is the reqirement for the overwhelming majority for a 'Britain with sunshine' holiday echoing the commedy sketch (was it Python?) of 'English beer, roast beef and tea like mum makes'. For many somewhere cheap and not 'too foreign' is the No.1 reqirement ahead of the culture of the destination. We aren't alone, the French are similar, the reason Club Med. was set up!
There probably aren't the raw numbers of more adventurous holidaymakers to sustain charters to far flung 'more interesting places' UK away from the Southeast and one or two departure points elsewhere by more than one operator. MAN would be one, BHX perhaps another, and as a long shot BRS if had a decent runway.