I thought it very strange when P&O claimed they were only reflagging two vessels. Not surprised with the latest announcement. P&O are claiming the British crews will be retained, so quite different to the Irish Ferries scenario.
However the latter are citing the need to have the vessels on an EU flag as the core reason.... Not sure how that works.
Pride of York which sails between Hull and Zeebrugge is flagged in the Bahamas. Likewise in the 1990s, Stena Fantasia was registered in the Bahamas, and her fleetmate, Stena Londoner. So I don't see how not being on an EU flag is such an issue.
In reality this is part of a gradual re-flagging of the whole fleet. First P&O Irish Sea in late 1990s, then Portsmouth (Pride of Bilbao in 2008), then P&O North Sea (c. 2010), and now Dover.... Mission Accomplished!