As you have realised, this WJEC Level 3 Diploma is not an A level - many schools are deliberately mis-selling it to 6th Formers as an A level, and it is not readily accepted by many top Unis for subjects like Law. Yes, you will find some that will accept it, but you will have to do some research/send some emails to be certain.
Please talk to your school/college about this - they are being deceptive in describing it as 'equivalent to an A level'. Yes, it is 'equivalent' in terms of UCAS points but it does not have the academic content / clout of an A level - and top Unis / those wanting specific grades know that and that is why they exclude it.