Let's try to work it through:
'Central south England' to Dover might take around 3 hours driving. Let's add half an hour for a break along the way. So a departure at 0700 gets you to Dover at 1030.
You need to check in at least 30 minutes before your departure and the crossing takes 90 minutes, so theoretically (if you got there exactly 30 minutes before your ferry was due to sail) that would be 2 hours. However there'll be a a wait for a ferry and time taken to disembark in Calais, so let's call that 3 hours. So you'll be leaving Calais at 1330 BST = 1430 CET.
You've now got a 6 hour drive ahead of you so (if you didn't stop) you'd arrive in Bielefeld at 2030. Putting in 3 half hour breaks at motorway services pushes your arrival time to 2200, so it's certainly possible to do it in a day (especially if there are two of you driving) but it might still be more relaxing to take an overnight break somewhere.
If you stick with your plan of staying overnight in Dover, this is definitely where you should stay:
http://www.brguest.co.uk/
I really loved it - and I'm not the only one!
http://www.tripadvisor.co.uk/Hotel_Review-g186313-d1028744-Reviews-Maison_Dieu_Guest_House-Dover_Kent_England.html