What do I need to do when I return with my family to settle in the UK; in terms of staying on the right side of US Immigration law?

You ask an excellent question. I'm sorry that I couldn't answer you sooner, as I just received your question today. A US "permanent resident" still can lose his status under certain conditions, and one of those conditions is if you abandon your residence by remaining outside the US for too long. You can file for a reentry permit with USCIS on Form I-131 prior to departing the US in order to show your intention not to abandon your US residence.  The application can be filed only by an applicant who is physically present in the US.  You still must maintain ties to the US such as filing US income taxes as a resident, keeping bank accounts, owning property here, etc. The reentry permit application is more cumbersome that it was in the past, as it now requires "biometrics" (which just means fingerprints and a digital photograph) and it takes more time to process than it used to.  Try to file the application as soon as you know you might need it.  An initial application should be granted for a two year period.  Best wishes for your family in the UK!