It has been mentioned already but it is worth making this clear: you do not need to do a law degree in order to become either a criminal barrister or a criminal solicitor. You have the option of doing any other degree you like, and converting afterwards. Some practising lawyers who did do law at undergraduate are glad they did, but to be completely honest, many of them (in my experience) wish they had spent that time doing something more enjoyable. All I can say is that it’s up to you, and that you will be at absolutely no disadvantage whatsoever applying for jobs in the area of criminal law with a conversion vs with a law degree.
For crime in particular, it’s worth noting that most people spend roughly the same amount of time studying that module at undergraduate law level as you would on the conversion course. So if you think you would enjoy studying something different at university - do that instead!