Take one step at a time.
Assets: House - £400,000, You could sell and move to a smaller place. Buying, selling and moving costs money and is not an instant solution. Nor is it a long term plan because even if you moved to a £200,000 house, it cost you £20,000 to move, the £180,000 you have left might last you 5 years - then what is the next question to ask.. You could put the money into a rental property and that could become your income... but net rental yields are between 2 and 4% so this is not going to guarantee a large income.
Business: I am assuming if this is not doing well, then it is unlikely that it could be sold. This is where your husband and his accountant / business advisor - or you, if he has no one else, need to sit down and work out where the business is, what can be done.
It clearly returned enough money to support your family and to enable you to pay off a house so it is not nothing. Even if it is contributing towards the load at this stage, it is bringing in some money.. however, at some point, it might be necessary to wrap it up - that won't be an easy decision, but if your husband could get a job elsewhere and the premises could be rented out, then this might be what needs to happen.
One of you can work and one of you can look after the children, this does not need to be one person all the time - share the load and see where your efforts are best spent.
Consider allowing the business to stop, defaulting on the loan and on the rent This is a huge decision and would prevent your husband from being a company director, getting future loans, etc.. so I am not saying this is a good idea - but it is one you should consider.
Then see how much money your husband can earn elsewhere. I also know that it is not easy and that the headline unemployment rate is masked by part time and zero hours contracts.
But number one priority is getting a handle on how much money is going out... I am sure you could stem that quickly!
Then work out if your husband is spending more money keeping his business open than he is bringing in.... and run some scenarios, it is hard to step back and work out what to do differently, but this is a very important step. It might also bring the realisation that in fact, there is some merit in the business but some changes need to be made.
You have been successful as a family before, you can do this again.. do it together.. I am sure your husband is suffering under the weight of this, share the load.