I have to say that while I have wild camped, it's not my preference when trekking. I do try to aim for somewhere with a pub that, with advanced planning, also does accommodation (it's nice to have a pint and something to eat, a shower and a warm bed for the night). I don't book because I usually do the walks during school term time, so accommodation is often not a problem and I'm not tied to a specific distance each day. However there are times when there's nowhere, or the pub (or sometimes a B&B or Youth Hostel) is full in which case I walk on another mile or so until I find somewhere quiet. I do seek permission if it's obvious who the landowner is, but provided you stick to the basic rule and leave the site completely as you found it in the morning, I wouldn't expect, and never have had any issues. If you strike camp before breakfast, then even if a landowner/ranger does spot you cooking breakfast, then there's nothing to indicate you spent the night there.
Youth Hostels are generally OK since most of them are licensed and do food these days, however the YHA's policy in recent years of closing the more remote hostels to pay for upgrades to city locations has diminished their usefulness.