The only time you'll have a problem gettting a spot on a campiste would be if it's a particularly hot weekend in a popular spot - nice coastal areas/New Forest/Lake District/Peak District etc., particularly on Bank Holiday weekends, but even then most sites will find you somewhere, just for a small tent. There's a bank holiday on 25th August, for example (if you'll be here then), and if the weather's good popular places will be rammed. Saying that, the weather's equally likely to be awful in which case you'll have no problem!
Wild camping obviously depends what area you're in for opportunities to do it discreetly. Saying that, I know of people who pitch up in the middle of roundabouts, so I guess that also depends on your own preference for how 'wild' you want it!