You should look into if you can practice as a mental health nurse despite being labeled as a general nurse. I've only done NICU as a BSN grad and I've been told and have seen that I can still work in the NICU with a general adult nurse UK license.
Look into the NHS jobs site (http://www.jobs.nhs.uk/
) and search for mental health positions and look at the requirements. There are also manager contacts on the positions and you could ask them directly as scenario, although I don't know how many would reply.
I'm hoping to go over in the far future as a neonatal NP--there's no degree program over there for it like we have nor is there even a board for NPs; it's a very new role from what I've read. So, the NMC really doesn't have any jurisidiction on what qualifies a nurse practitioner; it seems more like the manager hiring looks at your credentials and decides if you fit.
From everything I've read, the NMC phone people just read off a script, much like USCIS immigration (a US federal agency). They don't/can't/won't tell you anything beyond what's on the script. Try not to get too frustrated at them: they're just doing their job, but know that you probably won't get detailed answers specific to your situation.
As an aside: how do you plan to get a work permit in the UK? Are you married to a UK/EU citizen or are one yourself? I know it's hard to get one if you're not...
Best of luck! Please keep us updated on your progress as it's invaluable to other reading this
I'm planning to take the IELTS soon!