You could do a bachelors degree in mental health nursing but would then have to complete a conversion course (MSc) before progressing onto a more specialised path i.e. clinical, forensic, educational etc. This would involve potentially another MSc followed by a BPS Stage 2 qualification or professional doctorate.
So yes, it's possible but would only take you longer and cost you more. There's plenty of opportunity to gain experience whilst studying or between qualifications, you could look at bank HCA/support worker roles and work alongside mental health nurses.