From December 2020 NS & I will no longer send any prize money by cheque. Instead they will pay only into your bank. If they have your bank details you should be OK. If they don’t the easiest way to let them know is via their website (nsandi.com). If you don’t have a password for this site, you will probably have to phone them – good luck!
More details can be found here: https://www.nsandi.com/get-to-know-us/goodbye-to-paper-prizes