David,
Unfortunately your understanding is correct for many sites ion the UK. For better or worse the advent of the Internet and Advance Booking Services has meant that many people now make multiple bookings for sites well in advance. A number then decide not to actually travel and either cancel, or even worse, do not do anything. You will find the C&CC will only accept advance bookings with a deposit and this has meant it is often easier to get on their sites. The CC has not yet seen any necessity to follow suit and it can be 'chokka' on certain sites. Very frustrating for people. You will see a number of threads on the topic on this Forum. You can of course always try telephoning to see if there are cancellations but this often means making a decision at the very last minute, and if the chosen site is still full, it can sometimes be a lottery to ge a pitch for a Saturday night. Many people have taken to using the Certificated sites as means of avoiding these issues.
As Val has mentioned in France it is usually never a problem to find a site unless, you wish the beachside in high season.Fortunately that usually finishes by the middle of August and then you can take your pick. The one potential downside is that far more people in France tend to use motorhomes and as there is a large network of cheap 'aires' especially for motorhomes there is a chance that some sites will close. Some are converting a number of pitches for mobile chalets to try and keep occupancy rates up. But who knows.