Hi!
There are alot of websites that you can visit for ferry prices. When currently live in the Uk (have been here for seven years) and usually use the Eurotunnl for the crossing. You can go directly to their website and make a reservation, the earlier you do that,the cheaper it will be. If you miss your time slot, they will get you on the next available train. The eurotunnel is a 30 minute crossing, the ferry, about 90 minutes. If you have an American sized vehicle SUV or minivan, make sure you book a slot for the size vehicle that you'll be using. We always book for larger sized vehicles like for trailers/motorhomes,caravans because we use a luggage rack on the top of our minivan, there are height restrictions for different carriages.
London is always a good choice to visit. It's expensive though, and drivng within the city can be a bit of a pain, there are different zones within the city and you need a permit, otherwise you run the risk of a ticket. Best to avoid, park your vehicle and use the underground or public transportation. There are a variety of passes you can buy, one day, three days, that will enable you to ride all zones. The British Museum is free and you can spend days in that alone, other free stuff, google and you will be offered many options. Warwick Castle is the best castle to visit in the UK (my opinion of course), it's in Warwickshire (about 2 hrs from London), your kids would love it! You can buy tickets on-line,but the cheapest option is to buy discount tickets from ITT on one of the Bases over here. Be prepared for all kinds of weather, especially wet! It's really cold right now! Waterproof pants, boots, jackets, etc. If you wait for decent weather here, you'd never get to see anything!!! Not sure what else I can tell you. Really depends on what your interests are, how much time you'll have, etc. Many road side stops have typical American fast food, McDonalds, KFC, Burger King. I don't know whether you can take advantage of Base Lodging while you traveling. RAF Alconbury, RAF Lakenheath/Mildenhal are about 70 miles north of London, an hour away by train. You can gas up on the Bases, otherwise, gas is about .99 pence per liter!
Enjoy your trip!!
S.