London Area August Weather

You should take a look at actual day-by-day weather stats from the recent past. Averages don't tell you much of anything. Try wunderground.com, selecting History, then Monthly, then using the pull-down boxes to check out August and September in recent years. I've linked to September 2017, a month of pleasant temperatures but many, many days with at least a bit of rain.

I was in London September 8-17 last year, and it wasn't significantly rainy. It's just that every day it either did rain at least a bit or it looked like it was going to, so I carried either a hooded rain jacket or an umbrella (or both) around with me every day.

Unfortunately, these days you have no guarantee that it won't be unpleasantly hot--a factor when deciding whether you need an air conditioned hotel. There were a couple of 80F days in late August 2017.

I lived over there for a couple of months until late August, and we were fine just opening windows the whole months of July and August, amazingly few bugs & no screens on windows where we were. There's a humidity level that comes up a bit in the warm afternoon, that was time to take off any jacket we might have had, and there's a changeable, random rain shower that can pass through temporarily.It doesn't define your whole day, and there are often rainbows because of them.

When my daughter and I returned a different year, in Sept, we had no rain at all that whole week, with one cloudy day. When we went to the Globe Theatre, my daughter wore a dress that was not warm enough so they gave us some blankets to cover up while we watched the play (it's an outdoor theatre). You'll find most likely that all the times people talk about the weather being changeable, it's very temporary. Nighttimes can get cool in Sept.

If you knew exactly what the weather would be every hour, it wouldn't be the real English experience anyway. The not-knowing is part of its charm :} The variableness of it goes through less of a drastic range than we see here in the Northeast of the US, usually. The weather channel's web site can get specific with you before you leave.

It also depends on what you plan to do, if you'll be going from one museum to another, you'll be fine, the bus stops have roofs on them if it rains, and the last I knew, a few years back, they were working on most every overland coach having AC. If you're planning extended time outside, as with all travel, there could be a need to be flexible. Kew Gardens (and nearby National Archives) seemed warmer and more humid than any of the surrounding area, that's by design.
Have a great time, I've gotten an offer for grad school at Kingston U London, so I may be there in Sept too :)

Have fun,
-Alison

Summer rainfall figures are difficult to interpret as often as not in August they are the result of heavy thunderstorms, not a long spell of dreary drizzle.

Just to agree that September often is nicer than August. But you never know....

Check the forecast about one week before your departure and pack accordingly.

I'd opt for hotels with AC, though.

We honestly could get anything from snow to a heat wave. Forecasts are reasonally accurate a day or two ahead. Have a selection of bad/good weather activities and adjust as needed on a day-to-day basis... Much like people who live here do. Have fun.