Possible job transfer from TX to London...what do I need to know?

My husband and I have 2 children and one on the way. We have the opportunity to move to London for a few years but I'm wondering what we need to know to make it possible. I know London is much more expensive to live in but for a family of soon 5 and one income what amount would be necessary? Background. We are young and used to a non-extravagant lifestyle. I want to live somewhere safe but it doesn't have to be nice and new. So, what are good places to live that are more affordable? What are utility costs like? Are there any unexpected costs that you don't see in the US? Also, my oldest will be 5 next July so I'm wondering what schools are like and when they start? I'm thinking his offer will be around 50,000 pounds. Is that enough to at least get by frugally or should we ask for more? We won't have a car, is getting around with 3 children doable? Oh also, what is the income tax bracket like? One more thing. We would be delivering a baby there so how much does that cost generally and what is the medical care like. Sorry for so many questions, any help would be much appreciated. It would be a wonderful opportunity for our family and I'm really hoping that it can work out.