What about Dublin & Belfast to Liverpool & Birkenhead?
Compare like with like.
Add in additional mileage, add in the huge bonus of Swansea Dover Vs Fishguard/Pembroke Dover.
Arrive Swansea 0700 or West Wales 1230 / 1245, J42 of M4 at 0745 or 1445 !!
Add in on the likley requirement of an overnight hotel in UK on the way to mainland Europe.
Its much easier to leave Northern France / Belgium / Holland at a respectable hour and make a Sailing in Swansea at 2100.
Compare like with like, travelling with young childern there is no comparison.
Get some quotes and Go pick on Norfolk Lines and P&O and while your at it look at the cabin requirements on Stenaline (ex Harwich), Brittany Ferries (all routes), LD Lines (ex UK & Direct), Irish Ferries (Overnight Crossings to France) DFDS..........
Be adventerous and check out Moby Lines, Superfast, Minoan, SNAV, Color Line, Finnlines, Polferries, Acciona, TT Lines, Stena Line (Baltic Routes)........ guess what the Baltic, English Channel, Med, Adriatic
all have cabin requirements.
Once again why pick on a community based service.