HMO mortgage availability

HMO Licensing

I am currently selling an HMO in Lambeth, SE London. The buyer is saying she does not want it to trade as an HMO and the mortgage companies she has approached indicate they will not provide finance for HMOs generally. It is complicated further by the fact that Lambeth only decided to license all HMOs in their territory 18 months ago, and having inspected the property have sent no further correspondence on the subject. I have no paperwork to show it is officially licensed.
The buyer says she wants the property to be let as a standard residential property and not an HMO, so can I get it de-registered.
If I get Lambeth to de-register (assuming that I can get through their notorious administrative set-up and that they will) and she still does not complete, how does that leave me if the property has to go back on the market and the next interested party wants to use it as an HMO?
I am finding it difficult to believe that no one can get a mortgage on a HMO either, or that she can not simply mark it as standard residential. (There is a wall that I erected in the original living room to create a fifth bedroom, and which she says she wants to take down, so it would revert to its original 4-bedroom status).
Any help or advice you can offer before I do something that I later regret will be very welcome. For the moment I am putting the place back on the market, while I decide how to proceed.