I think Farrell's best suited to playing 12, alongside a real ballplayer at 10 (ie. Flood). Which is why he and Hodgson are going well at Sarries.
The problem is, England won't have that luxury. (This is all assuming, of course, that they are ignoring anyone old)
What is left is a conundrum at 10. Flood is likely injured, and therefore the premiership has literally 0 capped candidates. The options are: Farrell, Burns, Lamb.
To fit the Flood - Farrell model, it would be easy to replace Flood with either Burns or Lamb, because both are ballplayers who I think would work well with Farrell outside them.
However, the 6 Nations is not a competition known for the high quality of its rugby, and especially not Murrayfield. It's more about pressure and intensity. Personally, I think a Burns/ Lamb - Farrell combination would be a huge risk, since we don't know how Burns/ Lamb will handle the pressure and he'll have a debutant outside him.
With Farrell at 10, you can pick Barritt at 12. Which gives two benefits, especially considering the nature of the 6N and Murrayfield. You'll have the most reliable option at 10, undisputably. And outside him he'll have a debutant, but one who is his clubmate and who is also the definition of reliable.
For me, Farrell-Barritt is blatantly obvious. The question of 13 remains, and whoever Lancaster picks here will be either a gamble (Trinder or a half-fit Allen) or painful (Tindall or Banahan).