I've now seen some specific impacts on LHR, with ATC tightening up the regulation of arriving aircraft. This is specifically because of the global IT outage. The first figure is the number of flights allowed in, the second = 60 minutes / per hour.

05:40 - 07:30 = 45/60
07:30 - 08:20 = 40/60
08:20 - 11:00 = 36/60
11:00 - 20:00 = 40/60

And we can expect changes during the rest of today. 36/60 is tight figure for a Friday morning, it impacts T2 more than T5 due to T5 being BA's base and thus a lot of aircraft movement before 08:20. But this will typically lead to delays and later today cancellations. The latter may be weather since there is some cumulo-nimbus possibility today (and certainly will be in Mitteleuropa).

T2 had a serious IT problem over the last few days affecting back of the house baggage screening. Ironically that got fixed overnight, but there is a T2 baggage backlog, if you have any Aer Lingus or Loganair connections.