I would say, chemistry, Biology and Maths are the subjects you should have to keep your options open, because I know most medicine schools would want Biology at A2 and some would want you to have taken it up to AS level atleast with a Grade B. Chemistry a good option because almost all the science related fields ask for Chemistry and then Maths because its classed as a science and I would say its the less stressful subject, even though its still hard but its mostly application, so you dont have to memorise huge chucks of information like Biology and History. If you dont take physics, it won't make much difference if you wanted to go into engineering, as for leeds for example you can still do all there engineering courses apart from Mechanical engineering, but others unis will still let you do mechanical engineering as long as you got Maths such as Swansea University.