I had never heard of these premiums. A quick bit of research revealed these:
Crippling shared ownership premiums are holding back house buyers in Milton Keynes, complain would-be purchasers https://www.miltonkeynes.co.uk/news/...hasers-3347925
Shared ownership, at a premium... https://www.bbc.co.uk/threecounties/..._premium.shtml