In 2014 EasyjetHolidays partnered with TUI for charter flights, and it seems TUI claim to be paying £10 per passenger more than the low-cost carriers in airport charges, so they are pulling out of lots of airports in the UK:
http://www.travelweekly.co.uk/articles/20187/tui-considers-move-to-third-party-flightsThomson/TUI have had two 737s at Doncaster in recent years, but whether that will stay the same remains to be seen. Easyjet don't fly from Doncaster (or didn't) but they use it a lot for training on landings because it's quieter than LBA and has a very long runway from the days when it was a V-bomber base as RAF Finningley. My guess is that Doncaster needs TUI's business, and will probably make a good deal to keep them there.
Jet2 probably get a good deal from LBA as it's their main hub.
In the increasingly likely event of a no-deal Brexit things will get interesting. They'll have to make a very quick arrangement for flights, including maintenance and training requirements, for landing in the EU. Ireland is completely enclosed in UK-controlled air-space, so until agreements are in place flights from Dublin would have to climb in spirals until they reached about 30,000 feet, at which point they can fly across UK airspace as part of the international over-fly laws. Ditto on arrival at Dublin! That's a lot of extra fuel burn. The reverse will apply to flights originating in the UK to anywhere in the world, which are currently covered by international agreements with the EU. That's an awful lot of expensive bureaucracy.