I read an article recently (will have been one of BBC or the Guardian) in which they thought that Farrell, if he plays, was most likely to play at 12, because Flood was a shoe-in for 10. Apparently Flood is one of the candidates for the captaincy!
That wouldn't be a bad thing. Toby Flood was blooded at 12 for England before moving to 10, which is his more natural position. Farrell may benefit from playing in a less 'pressure' position before moving to 10. There is no doubt that at international level the number 10 is a huge pressure position.