For A-Levels I picked Law, English lit, Psychology and Polish. At this UCAS convention, I was told that some law universities like 'traditional subjects', and apparently these subjects are traditional (except for Polish).
I picked law partly to get a taste of what it would like to be in university, at my school we have been told that they have brought down some content from university in order to make our transition from A-Levels to university easier. I picked Psychology because I thought it would really accompany law in the sense that there is a reason behind the defendant's actions, and you can have a better understanding of cases. Polish (and that goes for all languages taken in A-level) is highly respected regardless of the course you are taking. A lot of universities like it because it shows initiative and the ability to effectively communicate with a larger group of people. English I took because it's a passion of mine, I also figured it would suit better than biology (Which was my other option).
I have also been told History is good to accompany with law (but I reallllllly don't like history so it really wasn't an option for me).
The thing that links them all together is that they are all essay based subjects, which show to universities that you have extremely developed writing skills and it's actually quite challenging because of the volume of work that is demanded.
I hope that was helpful!