On which side? UK CMA and EC will definitely allow it. In some respects they can't block it since they've allowed the IAG merger and near controlling QR position in IAG. They would have to target the ownership structure first. That said, they may require some minor remedy actions.
It might be considered a diplomatic overstretch for ACCC to be too harsh on approving the QR/BA/IB JV, so I suspect the most they might do it focus on specific routes. Even when combined, can't think that the JVs would form a dominant enough position on any route to worry the ACCC, especially given that there's ample competition (even on SYD-LHR). The benchmark they set with QF-MU and QF-JL was clearly routes where there were no/few other carriers e.g. SYD-PVG and SYD-HND.
What's possibly likely is QF going to ACCC for something with MH or CX ...