This is a valid question because when the affordable homes came out, only a few lenders were approved to give mortgages on them. So the question is more like can you switch mortgage to a different lender who was on that original approved list or can the mortgage be switched to any mortgage provider in the market. And, has anyone successfully switched a mortgage on an affordable home to either someone on that approved list or to someone outside that approved list.
To my knowledge all mortgages on affordable homes were provided by local authorities and funded by the Housing Finance Agency. This is because there is a claw back on the discount from market value affordable housing purchasers received and this must be repaid to the local authority of the dwelling is sold on again with a certain period (I think 10 years). Banks refused to lend for these purchasers because they did not have 'first charge' on the security.
However if you are outside this claw back period I can't see why you can't get a mortgage from a commercial lender.
But why do you want to? Are you not happy with the interest rate on your county council mortgage?