Warning - I am no tax expert, but yes, your assumptions are correct ...
BUT don't forget 'National Insurance' deductions, which on a salary of £40,000 p.a. would equate to about £260 a month, just below 8% of gross pay. National Insurance goes to pay for a state pension and 'other social security benefits'. However, access to healthcare here isn't dependent on paying National Insurance.
If you have kids, as I do, you are entitled to 'child tax credits', which for me result in approx £850 a year (the amount depends on how much you earn) being paid to us on a monthly basis - but you can chose to have it paid however you like.
All parents also receive child benefit, about £18 per week per child - to spend however you like. Don't know what the eligilbility criteria are - our kids are joint UK/US citizens.
As for spouses who don't work, I don't think there is any special dispensation.
Taxes here are generally pretty straighforward and usually, it's all done for you by your employer - not many people here have to do tax returns. The only things you have to do are the child tax credits and the child benefit - and it's dead easy.