Amazing that you have such a clear idea what you want to do at 14, I'm 17 and still don't have a clue! Just work really hard at your GCSEs especially for your sciences, then for A-levels you'd need to do Maths and Physics, perhaps Further Maths and DT if they're offered as well but they're not essential. Obviously if you want to do Chemical Engineering you'd need Chemistry. Then you'll do a degree in Engineering, ideally an MEng.
Have fun,