It's a shame your school doesn't do psychology!
If I were you, I'd go for the biology A-level, especially if you've had your heart set on law for a while- I didn't think I'd enjoy it easier but it was actually so fun for me. But I'd advise against jumping into any big changes straight away if you're having second thoughts about law, so sorry this probably won't be very useful but I'd just say do more research into law (you have summer to decide on A-levels anyway, right?), do some work experience (and you can use that in personal statements for a lot of other applications on the same vein) and if you decide on law I'd say go for the change, but criminology sounds like an amazing btec to do. Also, you said 'apparently' which suggests you should possibly figure out which unis don't accept it and if some that look appealing in fact do accept it, then you have nothing to worry about.