Planning a longer crossing is more challenging - but the journey can be immensely rewarding. Frequencies are low, so it might be difficult to fit in with your ideal holiday schedule, and the cost can be very high, such that you're still out of pocket compared to a short hop from Dover even after allowing for the extra driving and overnight costs of sticking to the land route. But the ships are generally far more comfortable and well-equipped, making it a genuinely relaxing experience that should really enhance your holiday.

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Brittany Ferries is the main operator now on the western routes, with services from Portsmouth to both Bilbao and Santander, and from Plymouth to Santander. The company transferred its routes from Poole over to the private-sector French operator LD Lines, more of a freight specialist. But LD is now working in cooperation with DFDS, on the shorter Channel crossings as well - all part of a very fluid situation in the past year in the pattern of ferry services.

Back in 2014, LD ran two return crossings a week from Poole to Santander, and to Gijon.(100 miles or so further west, getting you that bit closer to Portugal and NW Spain.)
But these services only lasted the one season - no sign of their being revived as yet.

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I have previously enthused about the novel LD Lines service from Nantes / St Nazaire in western France across the Bay of Biscay to Gijon in northern Spain (map below) - an interesting and much cheaper alternative to the premium-priced services to Bilbao and Santander. It's been part-funded by the EU as part of the 'Motorways of the Sea' programme aimed at getting freight off the roads, but is currently suspended - we await further details.
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With new tighter emissions regulations outlawing most of the existing ships, there is very little opportunity left to travel to Europe across the North Sea. The only service out of Harwich now is Stena's twice daily service to Hook of Holland. Further up the east coast, there are overnight services, every night, with DFDS from Newcastle to Amsterdam, and with P&O from Hull to Rotterdam and Zeebrugge. But that's it - no more ferries to Norway, or even Denmark. So the long route round, through Denmark, is now the only way to get your own car to Scandinavia: but if you're up for adventure, it's a fantastically rewarding experience.