As a shared owner if you decide to sell your share, you can provided you have notified NLM. You may need to get a valuation and advertise your home through a specialist agency such as SharetoBuy.
You maybe able to find a buyer through the sharetobuy website.  Once the sale is complete, the new buyer will continue to pay rent and service charges to NLM.

The link below has more information on shelling our home.

https://www.sharetobuy.com/guides-and-faqs/selling-shared-ownership-home/