Cross channel ferries

[AlpacaLypse]

I've been browsing ferry fares and routes this morning, and I've noticed how day trips including vehicle are often far far cheaper than a longer stay return. Is there any reason why I couldn't book two day trips, use one to get over there, stay for a couple of weeks, then come back with the other? If I use two different routes and ferry companies would they notice? I rather fancy touring for part of next summer!

I suspect you'd have to book the return leg differently. If you didn't take the England-France bit that might invalidate the France-England bit. But if you booked that as if the day trip was France-England it might work I guess.
I.e book the outward journey as England to France and back and the inward one as France to England and back.

I am no expert at all so may be wrong.

If we go touring we could easily end up coming home from a different country - say start by going UK to Netherlands, return via Le Havre or Cherbourg.

Yes, it's just that if you've booked a return fare and don't use the outward portion they sometimes cancel the whole thing. It should be in the ts and cs.

Why can't you just book one way? This summer, we went Folkestone-Calais on the Eurotunnel & returned Hook of Holland-Harwich on the ferry. I booked 2 single journeys.

Because OP wants a cheap day trip fare & they are returns