You're looking at a situation where it's genuinely hard to predict because the UK's ATC infrastructure has been under strain for a while now. The staffing issues aren't quick fixes - they take time to resolve. What makes it tricky is that some days things run relatively smoothly while others you get cascading delays, and it often depends on which specific airport you're using and whether there are any unplanned absences that day.
Since you've got flights coming up, the practical move is to give your airline a call a couple days before you travel rather than waiting until departure day. They'll have the clearest picture of what's actually scheduled versus what's realistic to expect. Also build in extra time at the airport - even if your flight isn't delayed, the general disruption means more people are rescheduled onto other flights, so terminals can get pretty rammed. It sucks but it's the reality of traveling from UK airports at the moment.