If hubby does get £40K/year he'll be alright, remember that translates to $80K/year.
I'd disagree slightly on this Matt - What you've said there is a direct Currency Equivalent, which is currently almost spot on with the $2 USD to £1 exchange rate.
However, 'earning' £40K is not the equivalent of earning $80K in the US in terms of the 'buying power that 40K brings you'
In the simplest of ways '£40K GBP only buys you a smaller proportion of requirements in the UK compared to what $80K does buy you in the US'
and I do mean the above in 'general' terms - obviously with scrimping and saving and careful money management, £40K in the UK can be made to stretch quite far.
For the OP - From the sums you've given it boils down to the usual evaluation - Are you prepared to give up/compromise enough of your personal lifestyle in order to ensure you earn more than you spend on a monthly (any time frame really!) basis when you are here?
This is a preliminary outlook you've given, and there are elements of costs and requirements you've not included as yet. However, on the flipside, there are elements of living here which haven't been exploered - such as a cheaper place to live both house size and regional ..
So best bet is to research as much of what you can find out, base it on personal information and use that to compare different aspects to see if it's worthwhile for you.
By personal info i mean things like the actual figure printed on a job offer (if when that appears) the actual rent amount negotiated etc - althought currently you do have a very good idea of what to expect.
Like others have said, maybe moving to just outside of London or another part of the country could be an option, Costs are relative no doubt, however much as I hear 'costs were balanced out as when living in the USA and then here they turned out to be much the same' is not the 'usual' case - although it can be common.
Good luck !
DtM! West London & Slough UK!