You get cashback on internet purchases if the website you are buying goods from is affiliated with Quidco.
Its 6% for P&O (although Quidco do take £5 a year admin fee but only if you earn over £5 iyswim)
I have had £23.50 cashback for my P&O booking so it was worth it, and also have had cashback in the past for things like insurance, books.. I always check to see if the company I'm buying from are on Quidco before I make any purchases!
I booked Portsmouth-Bilbao on Monday with P&O via the Caravan Club. Quote came in cheaper than with P&O direct, although I did book campsite as well so that may have something to do with it, and they only want £75. And they don't even take that until they confirm everything. Final balance 8 weeks before we go.
We went dover-calais last 2 weeks of august this year, only booked it in june and paid £150 for car/caravan and 3 adults and a child. Took 2 days to drive to spain (same as bilbao) and got to see all the scenary on the way! I would not have missed driving through the Pyrenees and over the Millau viaduct. No sea sickness either!