Hi,
Some good points raised on things to consider. I'd also suggest waiting a little while on a few aspects until you do get to the negotiating stage with your employer. If they include some aspects of what they'll cover and make it known to you at least you'll know. From there, you can start the process of evaluation. The most important factor will be to see just how much you'll actually be paid whilst in London - will they just continue your current salary and cover the increased costs ? or will they bump your salary up to help cover those expenses? until you find that out, it'll be a little pointless worrying about things at the moment as you might find they do cover them!
From that point also, you can then evaluate realistically your expenses and costs related directly to you and your family's lifestyle, only you know that!
Once you are at that stage, then this site has topics and discussions that probably cover everything you will think of already with direct experience from others, feedback and discussion as well.
To help you feel better, there's many Americans and Canadians here already! so you won't be alone as such! there's also plenty of Brits who like and don't mind USA people here too ! in my IT career, i've only ever worked for US software/IT companies in their London offices, and have met many many of the US staff who 'came over' in similar ways as what you'll have
Cheers, DtM! West London & Slough UK!