bucko its completely irrelevant what decision the UK Competition Authority makes because the ultimate decision lies with the European Commission and any appeals thereafter with the European Court of Justice, who have already ruled on the issue of Ryanair's stake in Aer Lingus which is what the UK CA is investigating.
And aside from being a waste of UK taxpayers money, it also sets a dangerous precedent because the UK are essentially sending out the message to other countries that it's ok to attempt to block the merging over 2 airlines in another jurisdiction simply because they overlap on some route into and out of that country. So because BMI flew to Dublin, it would have been acceptable for the Irish Competition Authority to investigate BA's takeover of BMI and attempt to block it if it found reasons to do so, even though the takeover would have limited impact on Ireland.
As things stand, the Irish government is against Ryanair's takeover of Aer Lingus. But had it been supportive of it, and the European Commission approved it, the UK Competition Authority attempting to overstep it's remit and block the takeover of 2 companies based outside it's jurisdiction could spark a massive bilateral dispute.
So to answer your question, I doubt that's why the EC extending the probe deadline.