they are fine to ride in general and I have used them a few times - London to Roydon bit that is. can be pretty busy at weekends with joggers, families, dog walkers, etc. so not cool if you want to blast along. this time of year will be less busy. if going in the week you will find it pretty quiet.
my normal London to Cambridge route goes via Roydon when on road so can send you a .tcx if file if you like as you can pick it up from there. it's not that interesting. but riding from London to Cambridge isn't really. nice little ride all the same