It's 3.5 hours by train Carlisle London, a comparison to air from Carlisle:
Travel Carlisle to Airport: 0.5hrs
Check-in: 0.5hrs
Flight block time: 1.25hrs (ish)
Collect bags, walk to station, wait for train: 0.5hrs
Train in to London: 1.0hr
Time to Spare ? ... Go by Air !