The FCO are just being overly cautious! I had a long, polite argument with them when my son needed to return to Thailand with only 4 months on his UK passport, even though his Extension of Stay/Re-entry Permit was due to expire before the passport expired. They even tried to make me buy a Emergency Travel Document, as they said he would be denied entry at the border.
Fortunately, I took the wise advice from this Forum, and he was stamped back in without issue.