Isn't Stanstead just a change at Peterborough job? Maybe the connections don't line up well, but it's hardly complex, unless the calling pattern at Peterborough is unhelpful.
Luton is a different story though.
More the one train per hour really, quite risky - believe it's 20 minutes from North East and 50 minutes from Leeds though which is fine on a good day, but the railway isn't exactly the most reliable thing around.
For the past few years now, I also travel from Yorkshire to London to get a flight. Its often cheaper and usually offers better connectivity. I also usually stay over due to my flight being very early. Unless your flying to popular leisure destinations In Europe, or Dubai and connecting onwards and NYC, then London offers far better connections.
Yeah it's a valid point from Yorkshire, in hindsight, keep forgetting Leeds/Bradford doesn't really have the connections and by the time you went across to Manchester, you could be in London anyway. I'm thinking more from the North East where we've got a bit of choice with Heathrow, Paris, Frankfurt, Dublin, Amsterdam, Dubai etc. I know quite a few people head North though because of the school holidays being different times to get cheaper fares.
Mind anyone sane would be getting a train to Kings Cross, walk for 5 minutes and do Thameslink though rather than sitting for 6 hours, give or take, via Birmingham.