Do what you love and you'll be good at it.
There's plenty of work in law for people who have a good grounding in science.
If the courses you're interested in don't specify particular A levels as requirements then that's because they don't require you to have them.
Choose subjects you love and where you have the potential to do really well.
Ignore the advice about doing extra subjects to compete - top universities are really only interested in your three best/three most relevant A levels and the additional entrance tests they ask you to do. Some do consider your GCSEs though so make sure you do yourself credit there.
There are plenty of ways to demonstrate a passion for law - reading, work experience, spending some time in a courtroom, following law programmes on TV and radio, websites about legal matters, finding out about the law in an area that's important to you, finding out more about legal issues that are current. These are probably all quite a bit more relevant the doing the "most relevant" A levels.
So do what you love, and enjoy!