I flew quite a few times on Ryanair when they had the STN-NQY route twice daily offered at good times of the time - always almost full or very busy 737-800
also flew on EZY too from SEN and back and loads were very good A319
Sadly they both gave it up - since then its been a pass the parcel=musical chairs on the LON routes......and still is it seems....
Also as a post suggested using the train from Cornwall - Fine if you have more than half a day to spend travelling - very nice views with nice new trains but your are looking at 5-6 hours journey time at the least, with only Exeter having very fast trains at 2 hours to London.
There is the sleeper train from PNZ and PLY to London which has a Govt sub.