Nursing education and qualifications in the UK are very different to how nurses are educated in the US.
A mental health nurse in the UK is an RMN and a general nurse is an RGN (maybe you could google those to understand the differences between the two.
To work in mental health in the UK, a general nurse won't cut it. You have to go the RMN route and be a specialist in psych nursing.
Similarly, a UK RMN cannot work as an RN in the US unless the RMN has evidence on their nursing school transcript to demonstrate they did the minimum number of hours (both theory and practice) in med/surg, OB, pediatric and psych during their training.
Hope that answers your question somewhat, but google those 2 things (RMN and RGN) and see if that helps.
The RMN course consists of 18 months generalist training and 18 months specialising in psych.