The majority of international passengers route via LHR, admittedly, dependant upon the time of day, LGW is 75 minutes on the coach to/from LHR, but, according to traveline.org.uk , SEN is an average of 2 hours and 05 minutes by public transport to/from LHR.
Now allowing for arriving at SEN one hour before departure that makes any departure from SEN an average of 3 hours and 05 minutes after leaving Heathrow yet, according the the "National Express 504" timetable, in a similar timeframe one could be well on the way to Cornwall and already in Somerset or Devon as one might be departing SEN.
I've said before that SEN has a future but "London to/from the West Country" it is not, the West Country needs a London Airport on their side of town ... people don't appreciate travelling backwards before they travel forwards and vice versa.