I would consider taking some advice from a tax accountant. It might be that this is taxable under capital gains tax not income tax, so various reliefs and allowances will be allowable but it won't be possible to say how much without more information hence that's why you might need an expert. The worse case scenario is that 40% will be payable so I wouldn't spend that amount and put it away in a high interest account until you know.
Sorry, last bit wasn't clear - has he received either an additional payslip directly relating to this share option exercise, or has the amount (the £200k) been recorded in any way on his most recent payslip that he's received from work?
What I'm trying to get at is that ideally the income tax and NI due (NOT capital gains, just income tax) would have been deducted at source by the employer, in which what you and your husband have received in the bank account would be the net amount, rather than the gross.
Suggest thinking about CGT implications (if any), once you have clarity on the income tax and NI part.
it could be a capital gain and therefore taxed utterly differently and depending on what proportion of the shares he owned there are some significant tax reliefs an IFA will not have the answers, a tax accountant will
a capital gain on shares will not come under the remit of NI or PAYE
And make sure you move it from your regular current account. Call up your bank and get it transferred to a savings account or some such for now. You do not want to risk the lose/theft of your current account card giving some criminal a lovely surprise!