TUI holidays are a tour operator - not an airline - they charter airlines to fly for their holidaymakers - such as using their own in-house airline TUI Airways which is an airline that flies solely for a tour operator (TUI or Cunard for instance) and does not fly it's own scheduled flights, nor it's own routes. (unlike BA BE FR EZY and Jet 2)
TUI holidays does NOT (usually) cancel its holiday flights (unlike say BA BE FR EZY or Jet 2) when there are affected by Tech Weather ATC or other Operational delays, but will operate them with a delay, or sub-charter another carrier to fly their pax as best and as quickly as they can to their destination resort, or to get them home if that is the case of an inbound.
Hotac and meals etc must be provided in the case of lengthy delays.
TUI holidays does (as stated by a poster above) cancel departures due to low-sales or consolidation of flights and resorts but this is done usually months ahead of the program starting.
TUI airways UK has had a pretty gruelling summer so far with bad delays around the stations from BOH to ABZ due to various reasons, coupled with 9 aircraft down in the UK TUI fleet due to the 737 MAX groundings (6 a/c delivered, and 3 more were due in service by end of May) plus retirements and standing down of about 8 of the 757 and 767 fleets - Most of these a/c have now been restored back to TUI for an indefinite period pending the outcome of the Max debacle.
Many ACMI operators and sub-charters have been utilised by TUI to step in all summer long to fill the gaps...
The costs will be approaching half a Billion £pounds or maybe much more........
Summer packages for 2020 are already on sale so the poor TUI planning Dept have a right old job now to decide what and where and when to operate, and with what aircraft they think they might have....