P&O Ferries are
Portsmouth to Bilbao
Dover to Calais
North Sea Ferry Routes
Northern Irish routes ( although only a few left )
And a route Ireland to Cherbourg.
The old Portsmouth routes are gone, contracts ripped up and no mentions of them.
Only the "money" making routes surviced the last round of cuts, hence Portsmouth went by by.
The OLD stena deal was a 60 / 40 split, stena being the minor share holders of that route. Within a few years and the Mergers and monoploy commision happy P&O re-branded and the final 40% was bought out. Stena still have market share on the Wales to Ireland routes and UK - Holland.
As part of the agreed deal on the Dover straits, the old Liverpool P&O freight routes and ships were transferred to Stena line, hence they increased their market share.
Hope that clears that one.