I am currently looking at my GCSE/A-Level options. If I want to go on to study engineering which subjects would be the most useful?
Engineering is fundamentally the application of science and maths. Within any field of engineering maths is the fundamental foundation.
At GCSE level you would be wise to concentrate on sciences together with product design or resistant materials.
At A-Level you may have a firmer decision about the path of engineering you prefer. At this stage you may wish to specialise in a particular science in addition to maths. (e.g. to pursue a mechanical engineering degree you may choose physics, for chemical engineering - chemistry etc)
It is always useful to have another subject in addition (if you have the time) to add breadth to your understanding of the world we live in.
If you are planning to study engineering a good basis on which to build on would be gained from studying Physics, however in order to ease your later studies a good level of education in Mathematics is also required.
Depending on the level you were planning to enter engineering at Technical Drawing, and Computing would be beneficial. In order to carry out almost any role in engineering a good grasp of the standard office computer applications such as Word and Excel required, and some specialist applications will be used, however you will be trained in these specialist applications either at university or in your role when it is required.
A good rigorous approach would be to look at the websites of the well regarded engineering universities and see what their requirements are for the course(s) that you might consider taking. However, I wouldn't worry too much about the fine detail, as once you have started a particular engineering degree course, it is usually feasible to switch to a different course after a year, as many of the first year subjects are common across all engineering courses.
I would also want to make sure that my typing was fast and accurate (for me, being able to touch-type is a must), as so much of modern engineering involves interaction with computers, from use of design software to communicating the results of field testing. It is vitally important that you can both understand and apply the art and science of engineering, and communicate effectively with those who might not be so gifted!