I believe, if you push back and park at your STD then you are deemed to have departed on time! You might well end up waiting an hour, over the otherside of the airport, for a slot, but 'No delay' will be recorded by the CAA.
Actually the CAA stats record take-off time and landing time, and a taxi time assumption is made based on the airport and whether it is a departure / arrival.
So you can't get away with pushing back off stand on schedule at 09:00, then taking off at 10:15, your departure is recorded on the CAA stats as 1 hour late as it relates to take-off time.
Remember, arrivals at uk airports are counted as well.
See Here, 5th table down