The requirements for entering the UK are found here.
Unlike some countries, the UK does not have a requirement of six months' passport validity when you enter. Instead, the guidelines simply state (for non-EEA nationals):
It must be valid for the whole of your stay.
Obviously that doesn't apply to you, since as a permanent resident you can stay as long as you want. I don't think there is any issue here, and you shouldn't have a problem.
(The issue here is ability to leave the country if and when required to do so. For someone who was only allowed to stay in country a limited time, an expired passport would be an impediment to meeting the legal obligation to leave--hence the validity requirements, and associated requirements like having a valid ticket home. But this is not a concern for a permanent resident).