My grandmother is from South America - Guyana, my grandfather is Jamaican, my mother is a mixture of both. However I was born in England and my father is Irish. What is my ethnic background? Help me please :)

You are mixed ethnicity, like much of the world. That means you can call yourself whatever you want. You forgot to mention your father's parents and grandparents? You could ask your grandmother more about her background to find out if she is part of a South American Indian group, or if she has any Dutch in her, or English, since the country was once owned by the Dutch and then the English. Also Jamaicans are often from other places, Jamaica being pretty small. You could write it all down in a little book, as a family story, and draw a tree.
For a gov't form, ask your mother what she considers herself to be, and hyphenate that with Irish.