Eating apples hold their shape, which means, once cooked, they are leathery and chewy - not ideal for a crumble.
Bramley is by far the better apple as it begins to stew more readily and you get a nice mixture of jelly and chunks.
Essential for me is getting the ratio right. Far too many apple crumbles you get in restaurants are full of apple with a thin layer of crumble on the top - so thin it begins to get soggy and merges into the apple mixture.
Let's face it the crumble is the nicest bit, especially with custard, so it's well worth making a nice thick layer of it