UK: do you book ahead at campsites or just wing it?

I'm planning a wee trip of 3 to 4 nights across Northern England. I intent to camp and having had a look at some potential campsites across my route I see that they either have very basic websites with minimal details or are hosted by the caravan club whereby the most basic pitch is "electrical hookup, small van" for like £40.

So my question is this :
Do I plan in some kinda detail where I'm going to be staying and phone ahead to book before setting off?
Or (and this is my preference), do I wing it and, say after x amount of cycling, look for the nearest camp site and just rock up?

For the second option, those of you more experienced than I, do you find this works? I imagine most places even in peak season will have room for one extra cyclist? If so, is the price they offer fair (<£15) or does the owner extort your desperate need for a bed for the night?

Finally, should all else fail, I'd be quite happy hiding behind a drystone wall. Anyone got any experience of this? I'd like to avoid the whole camper / farmer slurry assault .

Thanks 🙂