School holidays & summer crowds - London

Are you inquiring about the English school holidays? If so, school is not out for the summer until sometime in July. There is a half-term holiday of about a week at the end of May/beginning of June.

Thanks Leslie! The half-term holiday of about a week at the end of May/beginning of June is very helpful.
Now I'm thinking about other, non-Brit tourists mid-June. And wondering when it starts to get really crowded

Its always crowded in London. I’d stay with your London dates.

I am an ex-pat and I like to visit in June. There will be a week-long holiday (Whit-Week) for the schools around Memorial Day weekend (May 29th) so I tend to avoid that week. I do find the June weather is mild, the countryside is in full bloom, and the crowds are less. As for London, it is always busy, but I don't think of it as so crowded it is miserable. European schools don't break up for the summer until mid-July and go back in early September. Everyone in Europe is on vacation somewhere at the end of July/August. Avoid this time if you can.

The one that catches people out in London for crowds is the time around Ascension Day. Not a British holiday but a common one elsewhere in Europe. This is 18 May this year.