Depends very much where you want to go. If you're after Devon or Cornwall, near the beach in late July or August, you might need to get your skates on. If, on the other hand, you are looking for something in the middle of Essex for late May or early June, you can probably turn up on the day. I would suggest that, as soon as you know your holiday dates and where you want to go, get on the phone and ask about availability. It always seems a shame to me that the great advantage of caravanning, i.e. just drifting off where the fancy takes you, is gradually being eroded with booking being essential, deposits paid and even campsite owners asking for payment in advance (so you can't sneek off mid-week if the site doesn't suit).