I think it would be very difficult for someone (DECC or otherwise) to stop you selling at 43p, since that *is* the FIT as regards the current statute books, and 4 high court judges have ruled that it cannot be changed retrospectively.
If the supreme court ruled in DECC's favour, under the circumstances, it would be difficult to make a case of miss-selling stick to anyone selling at 43p, particularly if you make the customer aware of the ongoing court case and its ramifications.
However, more important than legal considerations, should be reputation. If you sell at 43p, and DECC wins, you will have a lot of disgruntled customers who will blame you, and that isn't good for sales or referrals. If you sell at 21p then whether DECC wins or loses you will have happy (or even happier) customers who are likely to give referrals for new customers. So which is more valuable - quick sales or long term reputation?