In the UK at least (apart from Scotland), wild camping is only legal with permission from the landowner (even in national parks), so most people use campsites to avoid the possibility of being driven off by a ranger or angry farmer.
When my SO and I did a multi-week cross-country trip we went for whatever designated camping sites was close by and pitched our tent there (except Berlin, there we went for a Airbnb). Though for us this mostly replaced switching hotels every other day. We then did local stuff around there, like canoeing, hikes, museums,... However, we did spent some nights in the wild, with camp beds (US: cots) placed next to the car. But when you don't know the general area, finding a good spot for that proved rather difficult. Entering forests with a car is usually verboten, and close to main roads there will at least be a lot of morning traffic; OTOH we had a pleasant conversation with some elder locals on a morning hike when we found a spot near a nice fishing pond in a small forest (yes, yes, driving there was erlaubt, not verboten). They only regretted not having some coffee for us :)
If you're considering going to Scotland and are thinking of that sort of thing, you should also look up the concept of a "Bothy" or "Bothies", which tend to be in mountains. They are a sort of refuge/simple transient accommodation. Generally small, quite basic huts in the deep countryside and remote places, where you can stay without booking and for free, but just overnight while you're on your way around.