As of 2:30pm, Cirium data shows the following:

• 232 flights have been cancelled departing UK airports so far today – equivalent to around 8% of all departures*
• 271 flights have been cancelled arriving into UK airports so far today – equivalent to around 9% of all arrivals*

However, do bear in mind that this is only looking at cancelled flights, and currently a lot of services have eight hour + delays which will inevitably result in a cancellation.