According to the OpenBanking API Spec:
The account name is the name or names of the account owner(s) represented at an account level, as displayed by the ASPSP's online channels.
Note, the account name is not the product name or the nickname of the account.
Comparing Plaid's screen scraping logic and the OpenBanking logic, NatWest's returned account name is incorrect. NatWest does not return the full legal name of the account owner and instead returns some sort of modified account nickname. Note that the name that NatWest returns matches the account name during the account selection phase or the OAuth login flow but is definitely not the account owner's name.