I was involved in repairing a number of Airey houses a few decades ago using the Leeds City Council repair system which basically involved forming masonry walls to relieve the faulty posts of their load bearing function. Option 1 dealt with the external walls only, and left the Party and Internal wall untouched on the basis that these were internal and therefore much less likely to deteriorate. The second option was to deal with all the walls (internal and external) and those repaired in this way were then considered to be conventional houses and therefore acceptable for mortgage lending purposes.
These houses a have all sold and some resold since that time and it therefore appears to be the case that there are lenders out there who will take these on providing there is proof that they have been fully repaired and that the necessary Consents were obtained and complied with. In these case PP was required, in addition to B Regs, as they increased in size due to the additional masonry outer wall
I also believe (but have not been able to verify) that some of the partially repaired houses were later mortgaged following the issue of a “Certificate of Structural Adequacy” by an Engineer, and I presume that this certification was only required to cover the mortgage period