Eh? Farrell's been perpetual first choice for the last five years, has been consistently the best option and has delivered every time.
The fact he's now shoehorned into 12 has nothing to do with the fact he's not quality, he'd be 10 if Ford could play 12 and he couldn't. He's never achieved much internationally at 10, but at 12 he's won a Grand Slam, whitewashed of Australia and gone a year unbeaten. Oh and he's scored 500 points for England.
The difference between Ford and Farrell at fly half you talk about is Jones.
Sure there was some improvement in the 2015 6Ns with Ford at fly half, but you could also attribute that to Joseph or Watson, or just not Twelvetrees and Burrell. And they still came second, losing to Ireland.
I don't agree that he delivered between 2012 and 2015 and I don't agree that he's been quality. And I don't care what he's done at 12 when considering him as a 10 and I don't care how many points he's kicked when judging him as a fly-half, not a goal kicker.
You could attribute the improvement in the 2015 6Ns to Joseph and Watson. I'm not sure why you would though; Ford scored as many tries as Watson and was instrumental in creating every single one of Joseph's tries. Ford was one of the last three people to touch the ball for 11 of England's 19 tries that 6N. He was pretty clearly a huge difference.
But if you really want to see the difference between Ford and Farrell, look at the 2014 AIs where we lost the games Farrell started at 10 and won the games Ford started at 10. Or the Australia games in the World Cup and the first summer test, where Farrell started and was moved for Ford both times, with both times resulting in the score line improving. Or compare the results in big games they've started. Farrell's 8 big games* at fly-half since 2014 have resulted in 2 wins and 6 defeats. Ford's 4 big games in the same period under just Lancaster gives you 3 wins and 1 defeat.
So its clearly not just Jones. There was already a big difference in what you could expect from the two under Lancaster. And that difference is why I disagree with the idea that he delivered and that he's quality.
In fact, the difference is so marked that I'd say consistently having George Ford at fly-half is one of the obvious big personnel changes from Lancaster's reign that has helped Jones achieve an unbeaten year.
*big game here defined as one of Ire, Wal, Fra or SANZAR in 6N/Tour/WC. No WC warm-up games considered.
I did a count of Farrell's big games for the whole of Lancaster's tenure and it comes to 8 wins and 10 losses. So better record overall, but a record that has got progressively worse with time (despite him clearly improving as a player). Strip out all games against the All Blacks and its 7 wins, 7 losses.