Ditto other replies re: your individual circumstances, but if you can avoid school holidays and even weekends, so much the better. It's a lovely idea to just go on impulse with no fixed plans, but depending on where & when you're heading, it's definitely best to book ahead if possible to avoid disappointment. If you're going to Edinburgh in August, for example, it's probably already too late to book! Apart from a couple off longer trips, we mostly do 3 or 4 nighters. We went to Northumberland last month, planned to go Wednesday - Friday, and held off booking (to see what the weather was doing) until the Monday, only to find Friday night (and all weekend) was already fully booked on our preferred site, so went Tuesday - Thursday instead. Probably best anyway - we don't want kids kicking balls at our pop top and screaming all day long

(I, as a child, was impeccably behaved and maintained a vow of silence on our family camping trips, of course

)
We've booked our Mull trip for May (including the ferry which may or may not be running, which could b*gger all our plans up, plus sites on the way there and back), because last May I only booked a site on Skye, and had problems finding somewhere to stay on the way back.
On the subject of finding sites, what do others do? I mostly decide where we want to go and then just use Google Maps to find conveniently located campsites, and then filter down to those with our requirements (EHU, showers/toilets/washing up areas etc.) I tend to book via the sites' own websites if possible or use Pitchup or Campsites.co.uk. When we started (2 years ago) I tried to keep to about £30/night, but now have accepted that anything less than £40 is more realistic. And you have to watch out for hidden extras, which really gets my goat, such as awnings (why??), or metered showers or EHU.