Indeed, if limited training capacity is the justification, why is it restricted to TCX who are almost all needing a full type rating ... while there are many very experienced type rated captains, some of whom are under threat of redundancy or significant upheaval, who would leap at the opportunity, and burden the training capacity far less, or TUI copilots who could upgrade, and be replaced by external type rated FOs.
This is gonna get really messy. Not doubting anyone's professionalism or skill but older people with a new TR especially if they've never flown a Boeing before is generally speaking, a bad recipe and one that breeds resentment amongst high time on type FOs. CRM issues will creep in if not handled carefully. Not a problem at some carriers but in a seniority driven, age-old UK company this will be complex. Also, in case of a downturn and job losses (unlikely I agree) I would imagine a former TCX Captain would have to be made redundant (instead of being demoted) before a high ranking TUI FO?