I live in Holland, and i can give you some info about a weekend wsbk or motogp.
If you like to go out and party, you'll better go to the motogp race. There is much more to do in the town of Assen the day before the race, during "the night of Assen". I am sure you'll like it !
If you go to the wsbk, you'll find out it is a little less crowded, and the things organized in the town, are not as big as they are during motogp. Wsbk is more for the "true" race fanatics, and cheaper !
If you travel from Hoek van Holland to Assen, it is about 250 km. And from Calais it is about 500 km. I would take the ferry from Dover to Calais, because that is much cheaper, and faster.
There are a lot of campingsite's around the circuit, and during wsbk you won't have a problem finding a place to stay. You can google them, and even reserve, but that is not really necessary for wsbk. Motogp is a different story.
Be carefull with speeding in Holland !
During weekends like these, police are driving undercover cars and bikes, aquipped with hidden camera's. And those bastards will try hard to ruin your weekend ! (trust me, i know) If they pull you over and fine you, you'll have to pay immediately, or the bastards take your bike.
I hope this can help you, and you'll have nice weekend
Ps the best spot on the circuit to wacht the race is on the hill between "ossebroeken" and "strubben". Cheapest tickets, but the best view !