The physical deeds to a property are no longer important these days, as almost all properties are on the Land Registry's computers and it's those records which are important in determining ownership. A potential purchaser (or, in practice, his/her solicitor) can check that your husband actually owns the property simply by downloading a copy of the title register for £3 from here:
https://www.gov.uk/search-property-information-land-registry
If there had been no mortgage on the flat he could transfer ownership to the buyer simply by completing the relevant form out of this lot:
https://www.gov.uk/government/collections/hm-land-registry-forms
However it would be unwise to try to do so without using the services of a solicitor and he/she would then get the correct form and ensure that it's filled in properly. (As well as the form filling, a legal contract needs to be drawn up between your husband and the purchaser, giving details of exactly what is being sold and the date upon which the transfer will take place).
However, as there's a mortgage on the property, the title register will show that the lender has a charge against it, meaning that he can't sell it without their consent. Therefore the first thing he needs to do is to speak to the mortgage lender, so that they can consent to the sale (and, once it's gone through, transfer the physical deeds to the property to the new owner or his/her mortgage lender).
It's also a legal requirement that he obtains an energy performance certificate for the property before putting it on the market. He can seek out an assessor himself or simply ask his estate agent to appoint someone on his behalf:
https://www.gov.uk/buy-sell-your-home/energy-performance-certificates
(The links at the top of that page might also be of interest to your husband).