There are points for both sides in this discussion. Yes, if you wish to get a specific site for a specific date then book as early as possible. That is logical. However, many people do not know where they will be going until close to the actual time, especially if they wish to go for longer than a weekend. The weather in the UK is so unpredictable that it is natural to wait and see what are the likely options. Should one go south, east or west for the dry sunny weather?? Also, if one is travelling a long distance from Scotland to the South West for example, it is no fun to arrive on Monday and be told to leave on Friday just so others can have their 48 hours of frivolity, and then skip their 10 miles back up the road to home. I fully accept that sites will be full over the school holidays and over Bank Holidays, and usually do not travel these times. But to find sites booked solid at weekends late into September does make one a little frustrated. It ceratinly takes away the original idea of caravanning as a 'freedom to travel' hobby. Now it is becoming more like Butlins every year.
By the way, although 1 week does equate to 3.5 weekends in horological time, it does not equate directly in access. 3.5 weekends equates more directly to 3-4 weeks of time that could be utilised. So the net 'loss' to the sites is not as clear cut as may seem.
We have decided for a number of reasons to limit our time on UK sites, and tend to head for Europe where access outwith the mad season of late July is a lot less hassle.