That has to be your call and without knowing the history far be it for me to advise you.
However, should you feel up to visiting accompanied by your bf I would suggest you regard it as a means of finding out how the land lies rather than an attempt to 'do business', so to speak, and it could be that once the ward staff can put a face to the voice on the phone they'll be more forthcoming. With regard to any questions your h may ask about the dc, I would suggest you prepare a number of anondyne accounts of their doings in advance.
Returning to the issue of your h's mental capacity, NHS beds are at a premium and 3 years is a very long time to be a voluntary inpatient which makes me think that, unless he is receiving some form if preferential treatment because he is 'one of their own', he will be sectioned if he tries to discharge himself in which case he doesn't have mental capacity to handle his own affairs and this, in turn, leads to me wonder whether there's a psychiatric social worker or similar who has facilitated a house call/visit from solicitors who were appointed to act for your h in the repossession proceedings.
If this should be the case there's no reason why you shouldn't proceed to divorce as you can obtain disclosure of your h's finances, and he'll receive details of yours, through the exchange of Form Es.
There's also no reason why you shouldn't proceed anyway, but if your h hasn't got mental capacity and doesn't have a 'litigation friend' it could be a very lengthy process with considerable delay before a decree nisi is granted.
As said before, you can file for divorce online and self-represent if you should need to go before a court - this isn't as scary as it may sound and I would imagine that, after all you've been through, you're more than capable of putting your case across.
Fwiw if you are on a low income or in receipt of certain benefits you should be eligible for a reduction in the current divorce court fees of £410 and you can find out how much you may have to pay here: www.gov.uk/court-fees-what-they-are