The first step you need to take in writing an essay is deciding on a topic. It can be difficult with such a broad assignment as there are several thousand possible choices.
The first step in deciding on a topic is thinking about what sort of poetry interests you. It will be difficult to write 3,000 words about a poet you find boring or repugnant.
Tactically, you want to search for a poet about whom you can find some information but not one so famous that the amount of information available might be overwhelming. For example, there is so much written on Shakespeare or on the major Romantics that you would not really have time to get a real feel of the scholarship available while very little is available about recent poets who are still building reputations.
To help you select a topic, I've made a list below of a few general themes or types of poetry which might interest you:
Humor/comedy: Several poets specialize in humorous or comic verse. If you enjoy reading humor, I'd suggest Lewis Carroll, Edward Lear, or John Betjeman.
Dark/Decadent Poetry: A few of the Victorians (especially Swinburne) and many of the decadent poets wrote about drugs, alcohol, prostitution, melancholy, and generally the darker side of life. Some English decadents include Ernest Dowson, Lionel Johnson, and Arthur Symons.
Female poets: Not all British poets are male. Many women ranging from Anne Finch and Katherine Philips to nineteenth century writers such as Elizabeth Barrett Browning and Christina Rossetti through contemporary poets such as Eavan Boland have contributed to British literature.
War Poetry: The experience of war inspired many British poets such as Wilfred Owen, Henry Reed, Siegfried Sassoon, and Rupert Brooke.