Your husband could theoretically accrue a beneficial interest in the new property depending on his contribution. If he is the higher earner and is contributing to the mortgage (whether named or not) then it could be construed that he is accruing a beneficial interest. Being married would make it easier to prove the connection but doesn’t automatically mean that.
HOWEVER:
The new house would be bought post bankruptcy and as such any theoretical beneficial interest would be “after acquired property”, which the Official Receiver can claim. This only applies whilst he is an undischarged bankrupt. If he was made bankrupt over a year ago, he should be discharged by now (automatic after 1 yr, if his discharge was suspended he should know about it). After acquired property is usually things like an inheritance received in the 1 yr period.
It might help to think of bankruptcy in three phases:
before the order is made - all the assets form part of the bankruptcy estate
The year of being an undischarged bankrupt - the Official receiver can claim most assets acquired in this period
After discharge from bankruptcy - the OR no longer has any call on assets acquired after this date.
The monthly payment (income payments agreement) he is making will last for three years, was agreed during the undischarged period but does not extend the bankruptcy past the one year automatic discharge.
It isn’t the potential new house you need to worry about, it’s the current one - you should check with the OR and get confirmation as to whether they intend to pursue your husband’s beneficial interest. If you sell, it makes it easy for them to claim the cash as they will put land reg restrictions on and the conveyancing solicitor will have to give them the money. It usually comes down to a commercial decision though - To be honest if they were going to I think they would have done so by now but you never know - as you say they have three years.
(I did bankruptcy work for quite a few years by the way!)