I'm pretty sure I used to be able to create lowercase pi characters using Composepi (as described on for example fsymbols.com), but it no longer works. My compose key works for other characters (like Composeaa for “å”), so what could be wrong?

I don't have /usr/share/X11/locale/en_GB.utf8/Compose (or ~/.XCompose), is that something which should have been installed/generated? There is a /usr/share/X11/locale/en_US.utf8/Compose; would it be sufficient to symlink that from /usr/share/X11/locale/en_GB.utf8/Compose to fix this?

$ locale
LANG=en_GB.utf8
LC_CTYPE="en_GB.utf8"
LC_NUMERIC="en_GB.utf8"
LC_TIME="en_GB.utf8"
LC_COLLATE="en_GB.utf8"
LC_MONETARY="en_GB.utf8"
LC_MESSAGES="en_GB.utf8"
LC_PAPER="en_GB.utf8"
LC_NAME="en_GB.utf8"
LC_ADDRESS="en_GB.utf8"
LC_TELEPHONE="en_GB.utf8"
LC_MEASUREMENT="en_GB.utf8"
LC_IDENTIFICATION="en_GB.utf8"
LC_ALL=