Within the field of aerospace engineering in the United States, there is some fluidity in the usage and definition of the terms describing the field; generally the term "aerospace" encompasses all of the aeronautical and astronautical engineering, covering the craft operating within or external to the Earth's atmosphere, respectively. The work done by aerospace engineers varies broadly depending on which job they hold- generally ranging from the people who do the up-front mechanical engineering design work starting with a clean sheet of paper to model the structural elements of the spacecraft to fit within the launch shroud of the rocket and survive the acoustic loads during the launch process to the people who test the fully-assembled spacecraft in large vacuum chambers that simulate the pressure and thermal environment of space to ensure that all of the electronics and mechanisms perform to their requirements. I gather from your question that you are interested in the discipline that people in the US refer to as aerospace engineering, and is often referred to as aeronautical engineering in the UK. Be aware that I have only a passing familiarity with the UK educational system, but examining the GCA Advanced Level course information available on Wikipedia, I recommend that you take classes in physics, math, and potentially electronics or design engineering to support your career aspirations. Your university education will require a thorough grounding in these basics and the entire field of aerospace engineering builds on a good foundation in math and physics- whether you choose to study the materials for building spacecraft, the science of spacecraft navigation, the design of rockets and thrusters to propel spacecraft into and through space, or the electronics to control and operate those spacecraft. Through studying the many aspects of aerospace engineering (with additional reading and internet research during your A level courses), I think you will identify a sub-field of aerospace engineering that is more interesting to you than any others-- perhaps it will be fluid dynamics, which is important to developing effective rockets for launching spacecraft, and then you will choose to take a course of study at university that supports your fluid dynamics specialty, or something else may grab your interest and you will choose that direction instead.
There are many sources of information on the internet that might provide you with useful background in aerospace engineering and information on the many career paths that are available; I recommend that you start with the following: www.careersinaerospace.com and www.aerosociety.com/Careers-Education Other sites I recommend are: www.esa.intl and www.nasa.gov both of which provide excellent information on current, future, and past space missions and they often provide profiles of people doing specific jobs in the aerospace workforce.
I wish you all the best in your future studies and finding the right career in a field that you enjoy, thanks for asking your questions,