These are the steps I made. I have no idea if any are redundant.
1) Overwrote /share/AbiSuite02.4/templates/normal.awt with normal.awt-en_GB
2) Overwrote /share/AbiSuite02.4/AbiWord/system.profile with system.profile-en-GB
3) Edited etc/init.d/locale.data to read en_GB.utf8
4) Created /etc/init.d/locale/en_GB. Copied /etc/init.d/locale/en_US/locale.data to new dir. Edited locale token in file to equal en_GB.utf8
After a reboot, the Abiword settings seem to be correct - Lang=en_GB, Paper=A4 Units=cm.
In xterm, if I type locale, I get a few warnings (I'll post a screen scrape from the CnM in a mo) but it generally looks correct.