Oxbridge and Imperial are the only medical schools who care about science A levels beyond Chemistry and Biology (and UCL actively state they favour a non-science 3rd A level) and this they now only go as far as a requirement for a 3rd A level being a science subject/maths (98% successful applicants at Cambridge have 3 or more science/maths A levels). Since the start of reformed A levels, there is only QUB and Manchester (I think) that require a 4th AS or A level (or similar), though Oxbridge will consider it if it is there. So if you have to do one, do the one you are most likely to get the highest grade in, as you are more likely to be looked down on for a B in Physics than you are for an A in Economics