There are numerous examples of BALPA, under a clear mandate from its members, preventing the further degradation of our working practices by opposing schemes such as this.
Completely agree with your point, yet another example of the good things BALPA membership brings.
A recent example of a similar situation is TCX pilots successfully preventing Ryan Intl' (US Airline) summer only Captains coming to fly G reg TCX aircraft and thus denying commands to time served guys who have dedicated their careers to TCX. Whilst not the same as the situation you propose the underlying principles are.
Again I'm in agreement DC, we probably aren't however talking about the highly unionized traditional charter carriers in all of this ala TCX, MON, FCA, MYT (or whatever they are going to call their almagamated selves).
On the flip side there are half a dozen other carriers in the UK that would view an arrangement like this with a degree of interest...mayby not what BALPA would want to hear....but then thats what choice is all about.
DC, on your final point, again, I am in agreement simply because the bigger charter boys simply aren't a viable option.