Jonny B Good,
As you stated, Prospect cannot withraw from the present agreement in order to extract a larger pay settlement. They are on notice to withdraw due to management breaches of the agreement.
However the present AAVA agreement is coming to an end. (date anyone?)
Prospect are well within their rights to not extend or renew it.
IMHO the AAVA agreement is hugely valueable to management, this is the biggest bargaining chip we have.