Well on your budget you only really have a choice of one ferry and that's Dover/Calais. Put a car on any other ferry at this time of year and you'll blow your £300 budget just on that.
Paris is always a good option from Calais if you've never been, on your budget look at
www.accorhotels.com, you'd get a F1 hotel outside the peripherique for 35 odd quid a night.
Reims area is also worth a visit, you'll find a cheap hotel on that website or a campsite on google no bother.