due to travel from the uk to south america tomorrow from gatwick and i
only have 4 months valid on my uk passport what do i do ?

You may be able to renew at the Consular offices in the country
concerned, assuming that there are British Consular Offices there.

In any event, you can always renew by post after you arrive.

Generally, Credit Cards are preferred for most aspects of
identification when travelling.

A friend of mine recently had his wallet stolen whilst in China, his
passport was one of the things stolen. He managed!

I don't suppose you are unique.

Don't worry ... You should be OK!

Have a great trip!

I once went into Mexico with a guy from the US

His US passport was well expired - yet he seemed quite unconcerned
- and it worked

It would however be sensible to plan your trip with a few visits to
consulates, just to show that you have made an effort.

Also, who knows, you might pick up some invites.
Have a good six months.