I'm going into mechanical engineering. When I was applying to uni's most requirements were maths (including mechanics), physics and another science. Of course further maths would be useful, however most unis don't require it.
I did Maths(+mechanics), physics and electronics at A level. I would change law if I'm honest. Change it for another science. Chemistry/computing/ biology/Further maths/ Electronics . I choose electronics as I found it easier than doing Chemistry as it does link with physics and I felt the content was easier to learn.
I know most places don't offer a level electronics so I would recommend Chemistry or Computing.
It should be the same for Aerospace, civil etc.