On an Apple UK keyboard, the # symbol is accessed by pressing ⌥-3 (pronounced option 3). Unfortunately the terminal application is only useable when option has been mapped to the UNIX meta-key, which takes precedence over “special” characters such as #. Thanks to this tumblr post, it is possible to work around this problem:
$ cat .inputrc "\e3": "#"
Unfortunately .inputrc is a bash-specific configuration file and this does not solve the problem for terminal-based applications.