Hiya! Another Year 12 Law-hopeful here.
I currently study Latin, History, Economics, and Japanese. All of these subjects are 'traditional', in some way (sort of); which Russell Group Universities tend to like. With that in mind, I would definitely recommend taking Maths. Not only will Economics be a great complement to it (in that the maths in Economics is really easy and should be a nice break for you), but it means you have a science, a social science, and an essay subject as your choices - it shows you're a well-rounded student!