London was great for my wife and I. We arrived early and stayed late to take advantage of the reduced hotel rate and spend two days touring on our own. January was cold but not bitter, was in the 40's (f) during the day and 25-30 (f) at night. We avoided any serious rain and had a lovely time of it. The wind off the Thames was a bit bone chilling though.
Start working on your passport now and avoid a last minute rush and paying an extra $150 for rush delivery. And as Robin said, the post office is where you get one. Just check your local Post Office as not all do them and some are by appointment only.
The flight is not too bad, we shuttled from our home airport to Atlanta and it was an 8 hour redeye from there to Hethrow. If you can manage it, I would highly recommend the trip.
We stayed at the Grange City Hotel (Official hotel #2) and were in view of the Tower of London, Tower Bridge and the conference was a short walk down the hill. The Grange St Paul was the Official hotel and was nice too but we preferred the Grange City Hotel as it was a bit cozier and quiet.
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"If you shoot for the stars and hit the moon, it's OK. But you've got to shoot for something. A lot of people don't even shoot." - Confucius