Authentication for 401 based failure is observed only on  iOS devices ,when the password contains £ character, it works for Form based authentication. Authentication for 401 and Form based is working fine without any issue on Android devices ,when the password contains £ character.

Coming to RADIUS:
Microsoft document , which is informing "Network Policy Server (NPS) does not support the use of the Extended ASCII characters within passwords"

https://docs.microsoft.com/en-us/windows-server/networking/technologies/nps/nps-best-practices#authentication

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This issue has been fixed in below NetScaler versions, request you to please upgrade to a version containing fix for the issue:
12.0 56+
11.1 56_3+

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Problem Cause

This issue is because we pass “as is” the password received from client, to the LDAP server. In some cases we need to convert password from Unicode-codepoints to UTF-8 to fix this issue.

Unicode codepoints for the range 0x00000000 - 0x0000007F pose no trouble: for characters in this range, codepoint == UTF-8 encoding. All ASCII numerals, English alphabets fall in this range. This is the reason why plain English usernames/passwords are interpreted correctly by the receiving auth(LDAP) server.
Conversion is needed for characters in the range beginning with 0x00000080. Umlauts (like ä, ë, ï ) and Arabic characters fall in this range. For these characters, codepoint != UTF-8 encoding. So we need to convert the codepoint to UTF-8 before passing on to auth server.