Instead of opting for the busy South of England ferries to the European mainland, it might be worth considering a more leisurely journey by taking a ferry crossing from the North of England with PO Ferries or DFDS Seaways.
PO Ferries has ferries from Hull to Rotterdam and Hull to Zeebrugge. Both routes involve an overnight sailing, but with two restaurants, shops and entertainment provided, the cruise to the continent can be part of your holiday. See my review of PO Ferries to Rotterdam.0
DFDS Seaways ferry crosses to Amsterdam. This again is an overnight sailing, but with three restaurants, shops and entertainments on board, you can arrive fresh in the morning ready for onward travel.