If you want to be a barrister you have to do this course. If you want to be a solicitor it is best to do the law conversion too (although if you need a student loan doing the combined law conversion with SQE1 masters for the masters loan with someone like BPP may be best). Even better is during your non law degree apply to law firms who recruit years ahead from undergraduates for vacation schemes and training contracts and if you get one they pay for all the courses and pay you after that during your 2 years of being a trainee solicitor with them (if you want to be a solicitor).
What a law conversion means is the core modules of a 3 year LLB done within one year so basic important subjects like contract law, tort, criminal law, public law but not some of the additional subjects I also did on my 3 year LLB.
You apply via the Central Applications Board (which is like a UCAS for law) and you can apply from September of the last year of your first degree or a bit later if you prefer)