Okay, this is my completely lay-person non-immigration expert opinion- just my thoughts on the subject from having been a US permanent resident for 13 years and going through I-90 hell and back (guess who still hasn't received her renewed green card that she filed for 3.5 years ago? But haha, I don't care because I'm a citizen now- take that USCIS!)
Are you planning on staying in the UK permanently, or do you want to go back before your two years are up? (Since you can only be out of the country for one year [or two years if you've got a re-entry permit] before losing your residency.) If you do plan on moving back and your green card just happens to be expiring, that's quite a sticky situation and I have no idea how you'd go about filing an I-90. (Also, even if your GC expires, your status doesn't expire with it- you're still a PR, you just don't have any *proof* that you are.)
However, if you plan on staying in the UK for longer than 2 years, it doesn't really matter if you manage to renew your green card or not. Sooner or later on a trip back to the US, the immigration officer is going to notice that most of your time is spent out of the country and will ask you about it, and once ascertaining that you've moved to the UK he/she will take away your green card because you're not entitled to one anymore.
There is an exception to this rule (especially when pertaining to eligibility for naturalization) but that has to do with say, a permanent resident getting transferred to a different country by his/her American company, serving in the military overseas, or something of that nature. Of course, I've no idea if that is your situation, but I'm just telling you what I know.
If you do end up losing your residency and want to move back to the US one day, there is a little-known returning residents visa you can get (the info's on the Department of State's website, not the USCIS.)