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.
Good luck
Also, if you plan on law, just get better at public speaking now as you will have to at some point anyway.