Well this is a hard question to answer. It depends on what you mean by *comfortably* a lot of things have to be taken into account...
1 - How large you want your house to be?
2 - How many people are moving, and how many will be living in the house?
3 - What kind of work are you going to be looking for or resources will you be depending upon?
4 - Where do you want to live in the UK?
5 - Do you want a garden, pool, garage or jacuzzi?
6 - What would you say the average amount of money that you spend in the US is at present for foodstuffs, groceries and other everyday items?
7 - Will you be driving or travel via public transport when you journey?
8 - How much time and money will you devote to leisure?
9 - Will you be going back to the states regularly?
10 - Will you have to pay money for child support or other constant outgoings that you have to account for?
Sorry, but these are just some questions you'd have to answer before you got a realistic estimate to your question.
Personally we spend £600 a month on rent at the moment; we're living in a one bedroom flat/appartment at the edge of greater london, we have a garden, and a parking space. Then you have phone, gas, electricity, water, cable connection, council tax, food, travel, entertainment (TV, movies, theatre, books, games etc.) that you will have to pay for too.
As I said at the beginning there are so many variable factors that you have to take into account that all this is probably useless. Sorry, but with the information that you've posted that's as much as I can help.