One of the best things about camping is the ability to be spontaneous.
Sometimes we decide on that day, sometimes we plan a week or so ahead then wait for the weather. The kind of site we tend to use don't always require an advance booking.
Having a trailer helps too, as its prepacked most of the summer.
It sounds like you're all a bit more organised than ourselves. We seem to need several hours just gathering all the stuff together. Maybe if we pre-bundle all our gear together we can just grab and go when I get back from work.
We always tend to book a day or so in advance as we leave our destination to the last minute based on weather forecasts. Also, we will generally book just the first 2-3 nights of a 4-5 night stay in case the weather turns.
We nearly always go EHU. Have had problems on bank holiday weekends. Jubilee weekend was a complete no no if you hadn't booked. But generally, if you're out of school holidays and its not a bank holiday, you can generally leave it until the day before or the morning itself.
My sister tends to always pre book, usually for an entire week, but ends of leaving early due to the weather, thus losing as much as £50.
------------- MAY 2017 - Loch Ken, Scotland
JUNE 2017 - Sango Sands, Durness, Scotland
AUGUST 2017 - Balloch O Dee, Galloway and Invergarry
SEPTEMBER 2017 - three brief trips
OCTOBER 2017 - hopefully one final trip before Halloween
If you intend to arrive at a campsite and ask for a pitch on spec. it is wise to appear polite and cheerful, despite appearing tired (and unable to go on much further).
Don't be surprised if there is no space and you have to move on, and don't show aggression if that happens.
If the campsite looks fullish emphasize how small your tent is and that you could fit onto any bit of green anywhere.
A regimented and organised campsite is less likely to take you in than a more "chaotic and happy" one.
Say that you only need to stay one night or perhaps two as the campsite may be able to fit you in on a night by night moving you about basis. I got put in the "garden" of a caravan for one night as the warden knew the owner would not arrive that day, but was moved to the edge of the kids play area for the next night. They were the only tent spaces in town those nights.
Expect the booming voices of dragons shouting across car parks "Complet, we are full!!!!!", and laugh about it later.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.