Google 'non standard construction mortgages' and you will find several companies that appear to provide what you are looking for.

You will have the same issue with Buildings Insurance, as most companies won't provide it. This is something you should look into, as a non standard Buildings Insurance can be a lot more expensive, than a standard policy.

If you are buying the flat make sure you research about the construction of these properties and whether you feel that investing is in your long term interests. From memory, I am sure that I have read about problems, where people have bought their flats and have regretted it. i.e some flats owned by council get renovated, with those that are privately owned not renovated, causing a problem if you wanted to sell/value of flat. Once you own the flat, if there is any ongoing maintenance to the block, you could be responsible for a relevant contribution amount.