In 2013 my experience was very similar to yours: my longest distance was 400k. I slept in the controls the first two nights, then switched to B6B and hotel because I was experiencing Shermer's neck and thought that a proper pillow would have helped (it did, after all). I hadn't booked. In Barnard Castle, the volunteers very kindly found a B&B for me, phoned to the host and explained the situation. She accepted me even if it was very late. I'm still very thankful to the volunteers and to that lady! In Woodhall Spa I found a hotel by myself.
It can turn out to be more expensive than other solutions, but I didn't waste much time. Furthermore, I use to visit the toilet frequently while sleeping (to be precise, I wake up, go to the toilet, get back and fall asleep again within microsecs) and a B&B/hotel room has a clear advantage from this point of view: in a control you have to put something on your feet (this subject is already being discussed in other threads), get decently dressed if you sleep naked to give air to those parts of your body that the saddle has irritated, cross the dorm, the corridors, find the toilet and then get back, try to remember the position of your bed, undress and finally realize that you're now completely awake and it'll take you a while before you fall asleep again.
I repeated the "no booking, just look around when you're really tired" strategy in my country last Spring on a 7-day-long audax, and it ran even smoother, because I had no linguistic/cultural obstacles. For cultural obstacles I mean that in my country I can predict accurately the price of a single room just looking to a hotel or B&B from the road, while in the UK my error is much larger. In my country, just by reading the names of the towns the route'll pass through, I can predict where accomodation will be available, in the UK I'm a foreigner and names don't tell me anything.
It's good to have plans, but it's better to be ready to modify them as your ride goes on.