FCA isn't an 'option' as such. They have firm contracts for the 787, as a launch customer, and also with GE for the GEnx.
Monarch have specified the Trent 1000 on theirs. The two would be incompatible. As TUI are currently doing a fleet renewal exercise, which will include the Thomsonfly (and therefore First Choice) operation I would think it is completely unfounded for several good reasons.