you can switch an affordable housing loan to any lender , so long as there is sufficient equity in the property to repay the existing mortgage plus the councils clawback. ( as you obtained the property at below market value, the clawback ensures that people are not using the affordable housing system to profit in the short term from selling the property at market value say a year after they purchased it at discount).
At the moment the only lenders doing affordable housing mortgages are BOI, EBS and IIB. In order to switch the mortgage to another lender, the council have to be fully discharged from any interest in the property, so the claw back that applies if you sell the property, also applies if you wish to re mortgage. apart from the lenders detailed above, no other lender will allow the council retain an interest in the property ( i.e the claw back).
work out the market value of your property, if the balance on the existing mortgage plus the claw back is less than 90% of the current value of the property you will be able to re mortgage with any lender. its worth noting, that councils always say to people, " you don't have to repay the claw back to re mortgage" however they seem to not take into account that lenders with the exception of EBS, IIB and BOI, will not take a property as security with the council retaining an interest by way of a clawback for the first ten years.