You don't need 6 months on it to come back to the UK. If you renew with up to 9 months' validity left, they add it on to the 10 years so you don't lose out. You might as well renew in the autumn after the summer rush, then it's done and you don't need to worry about it.
The reason some countries require you to have 6 months extra on your passport is that that is the normal length of stay on a visitors visa. They don't want a scenario where you could be in their country as a visitor and your passport runs out, in case you have difficulties getting a new one and leaving you in an immigration legal limbo.
I once had a friend whose father got refused entry by US immigration whilst going there for a medical conference - but he only had weeks remaining on his passport and this was about 20 years ago. You will have well over 6 months remaining when you go there. Why not see if there is anything on the US Embassy's website about duration remaining on passports, print it out, and take it with you?
Have just booked to go to Florida next easter, we fly back 1st may (arrive back 2nd). I know that you're supposed to have 6 months 'extra' on your passport, but not sure why? Anyway, my passport expires on 19th November, so a couple of weeks clear of the 6 month thing. Will all be ok with American immigration? Or should I err on the side of caution and get it renewed before we go?