Is it less crowded in the middle of Sept. in London? Planning first trip to London and we are very flexible as to month of travel. Please adivse.

Sinkevich - I think you find crowds in London during the month of September. I know of many Australians who now visit UK in September/October, which used to be a shoulder season, but hey, tourists soon learn which month to switch to.
I have found the weather in September, can sometimes be warm, but it seems to be very changeable in late September/early october. I remember the last time I was in London, it was September, and people were wearing summer clothes. I then drove north to Scotland, and when i returned in october, it was cold, and jumper weather.

Since you mentioned you are flexible as to month of travel I would suggest November. Obviously, not as warm as Sept., but less crowded (tourist wise). I've gone to London the last 3 Novembers and the weather has been fine. And I live in So. Calif. I prefer the cooler weather and fewer tourists.

I was in London last year the first week of September. There were tourists but I definitely would say that it wasn't crowded. I didn't have to wait in line at the Tower of London to buy tickets, which I've heard people often have to do. No where did it feel crowded, it did get quite warm, I would say low 80's,

September is less crowded than July or August. The weather is better than November. October might be a good compromise.

I've gone in both early November and late November - there is a difference weather wise! just a few weeks, but the leaves are about gone and it's damper and colder.

in early November it's still a bit green and not really cold.

No, of course London isn't less crowded in September (presumably you mean than July or August).

London half-empties in August. You can get a seat on the Tube, the roads are devoid of traffic, and you can eat at fashionable restaurants without a year's notice. Taxis are available easily, even when it's raining. On the other hand, theatres are at their dullest (who's going to waste original material on a bunch of tourists?), there are virtually no interesting art exhibitions and the recent boom in public lectures and debates goes on holiday like practically everyone else. And it rarely rains.

Touristy things and places may get fuller in August, though. Impossible to understand why: our weather's at its worst in August (London isn't designed for global warming summers), our life is at its most boring, and the parks look horribly parched. We get surprisingly little rain in Britain and have few lakes to hold rainfall: what rain we get flows quickly through our short rivers into the North Sea. So it's environmentally unacceptable to water public greenery artificially in midsummer.

All changes when we all come back from our holidays in early September and the weather breaks (traditionally throughout the late August holiday weekend). London reverts to its usual crowded, interesting, climatically tolerable and green self. I doubt hotel prices come down much: London also reverts to its role as Europe's commercial capital, so what the hotels lose in Japanese matrons they gain in Frankfurt bankers. But those Frankfurt bankers don't go to the Tower of London, so many of the tourist honey pots empty out.

Getting into restaurants or interesting plays and operas starts getting tough again - and stays that way till the end of June. The Tube goes back to its usual state of overcrowding. But no doubt the audience at the Changing of the Guard thins out.