Wait until 16. Investin isn't great imo, I feel like they just take excessive money for what sounds like not much actual law etc.
Get good grades, start LNAT prep in Y12 September as a really good score in that helps get into good unis which have a much greater career bearing. Also helps you practice logic and reading which is good.
Go to a Crown/High/Magistrate's court near you and sit in on some cases - that will show you real life stuff and you can talk about your 'experience' when applying for internships/unis/WXP later on.
If you want some law WXP, email some law firms near you well in advance (at least 5 months before you want it to start) and ask nicely. Contact friends/family if relevant. Don't bother with corporate part at this stage.
Law with language degree sometimes more useful than straight law for top firms, so for lhttps://www.thestudentroom.co.uk/showthread.php?t=7392538#aw, try at least one language A-level with the usual like English, History, etc.
Also, if you plan on law, just get better at public speaking now as you will have to at some point anyway.