July 20th might not be too bad, aside from the private schools most of the state schools don't break up until the 21st and most Surrey schools have scrapped their trips to Thorpe this year due to costs, we tend to make our annual trip to Towers the next week for that reason, August is much worse, especially the further you get through, the more bored the kids get the more desperate parents get and they tend to shell out for theme parks as a last resort later in August more than they would in July.
May half term will be hit and miss, if the weather is crap it won't be too busy, if it's good then it'll be a nightmare, Monday or Tuesday of that week will be quietest, but still busy compared to Towers in July.