Most A level English courses do a Shakespeare, so you could read one of his, or alternately watch a couple (or if you can, attend a live play if there are any local to you). Emma Smith’s This is Shakespeare is a great book to give some creative readings of many of his major plays, and you can also use her as a critic in your essays (I did AQA, which encourages the use of critical quotes)
You could also find out what set texts you will be doing and read something else by that author.
If you want something more hardcore, the Odyssey and Iliad by Homer are two Ancient Greek epic poems which are foundational literature, as they are frequently referenced in texts both old and modern.
Some more texts you might fancy:
Picture of Dorian Gray, A Raisin in the Sun, Far from the Madding Crowd, The Handmaid’s Tale, Revolutionary Road.
To be honest, I feel that with English any text you read will be helpful, as long as it is ‘literary’ (and even non-literary texts could come in useful in helping you to understand different genres or archetypal characters).