I have just had some very good news about getting our boats out from Harwich to Hook of Holland for the masters less than a one hour drive to the event.The cost from Richard at RB Sports travel https://rb-sport-travel.co.uk/
they have arranged several Ferrys for me and offer a great service Richard sails and knows exactly your requirements as a dinghy sailor. Prices vary subject to vehicle dates and times as well as for ferry company but mine worked around the £250 return booking early! Spaces and cost will only increase as the time get nearer to the departure date.
My understanding is that RB Sports can do even cheaper crossings with other alternative routes to Europe. All prices are subject to your individual requirements at the time of travel. The earlier you book the better chance of getting a good price. The return prices start at around £200 for standard Car, trailer and Boat without a cabin from other ports in the Uk to France.