@MaidOfAle - the thing is, OP can prove consent was originally provided by dad, back in January, and then withdrawn for no valid reason (other than CMS having been applied for).
If C100 for SIO is raised with exemption from MIAM (citing financial abuse) today, this could be heard within the OP's timeframe (ie before half term which is usually end October?). It's a risk, but I would imagine it's worth trying so as to remain on the correct side of the law (even though consent has already been given).
Like I said down thread however, having had lived experience of a similar situation (which was in fact somewhat more unnerving as parent A + child were travelling to their country of origin, the parent (B) who contacted border police was given a serious grilling by several officials as to why they would not grant permission for the holiday), it's an awfully difficult one to answer when the person attempting to prevent the travel has absolutely no valid evidence that there is a serious enough flight risk - particularly if the person travelling with the child has all the necessary documentation (passport, BC, marriage and divorce certificates (if relevant), a home, mortgage / rental agreement, job, wider family, or even, other children, and that the child is 14 and looking forward to their holiday...