Up the top where it says 'Tactical Update' ... if you expand that out it gives a list of the main current issues in the system and it has the text there that KARFA quoted above

It's not the most user friendly of sites , not really targeted at the general user.... but it's useful to see what's going on.

On the arrivals board the issue at LHR is that in the early morning there's lots of long haul arrivals which are not subject to these slots and they arrive at Heathrow anyway ( unless the forecast weather is truly awful ) so they are arriving relatively normally but short haul flights from nearby destinations like yours get subject to a take off delay to control the arrival flow if Heathrow ( or other airport ) has to limit things because of weather or other reasons