If you no longer live in the UK, then you should renew it in the country that you live, but you can renew it when you are back.  I would double check with your local embassy as the laws have become stricter than they were 10 years ago.  I had to get an emergency passport in London 10 years ago because i lost mine and was flying that afternoon.  I went to petty france and was in and out in 2 hours.  I only have 4 months on my passport and I thought I needed 6 months to go to the US but was only told this the day before.  I called the passport office and was told it would take a minimum of 4 hours, which would have meant i would miss my flight - but they would not do anything about it.  It turned out I only needed 90 days, so it was all good, but basically, it is much harder than it used to be.