I finished my Lejog on the Sustrans route on the 22nd of this month and I can honestly say that the biggest stress of my trip, was not pre-booking hostels/B&B's/hotels.
Whenever I arrived in a town or when it got towards 730pm which was my time to find somewhere to bed down, (either wild camping or paid accommodation.) There would rarely be a campsite near by, but mainly hotels/B&B's etc.
Lots of those places were fully booked and it would leave me having to cycling 30 minutes to 1 hour in the pitch black usually in the wrong direction to travel somewhere with a room available.
I'd say 50% of the time, I'd find it hard to find somewhere and the other 50% of time I either fell lucky with a decent hotel in a town or I was that remote I could just set the tent up without having to worry about being bothered by people mid sleep.
If I were to do it again, I'd probably do it the same and not pre book, but not pre booking does create quite a bit of stress if you leave it till 730pm like I did.
The advantage of not pre-booking though meant I could travel further each day and was not limited to only traveling the distance of the next pre-booked accommodation. Although it could work the other way and mean you have to travel further to reach your bed...
Specific board for this popular undertaking.