According to Wikipedia, modern English apple pie usually uses Bramley apples.
Bramley is a cooking-apple variety associated especially with Britain. Its high acidity and firm, sharply tart flavor make it useful in pies because sugar can balance the sourness while baking develops a concentrated apple taste. The fruit also softens during cooking, giving the filling a traditional spoonable texture.
Modern English pies commonly arrange thick layers of sweetened Bramley slices in a dome. Apples shrink as they cook, so the domed filling helps compensate for that downward movement and reduces the chance of a sunken center. The pie is then covered with shortcrust pastry, often made with butter or lard.
Bramley is not the only apple suitable for pie. Wikipedia also lists varieties such as Braeburn, Gala, Cortland, Empire, Northern Spy, Granny Smith, and McIntosh among common cooking apples. The wording “usually” matters: English tradition favors Bramley, while American bakers often choose other tart or firm varieties.