Basically vertical integration of businesses and consolidation within the IT sector meant that the UK wound up with 2 or 3 major players, with in house airlines, and clearly the business needs to keep as much of the flying in house.
Wind the clock back some years and you had several small / medium sized holiday companies buying capacity from various carriers. MATO (Midland Air Tour Operators) set up by Doug Ellis flew between Birmingham and the usual Spanish destinations using Spantax among others, and later Carousel Holidays became a fairly large regional player using Aviaco among other carriers. There will have been similar operations from across the UK regions.