I have cruised with P&O, and although they don't have real ale on tap, they do have good quality bottled beer. I have had Marston's Pedigree, Shepherd Neame Spitfire, and Speckled Hen. Price varies from about £2.30 to £2.50 per 500ml bottle.
I had same beers on P&O but it was only available freezing cold so it defeated the object of having good beer.
I think you'll need to rely on bottled real ale. The stuff from the barrel has a limited life, and more importantly needs to sit very still.