With regard to the right to an Elective CS, you DO NOT have the right to one. It is important to stress this, and the point that the NICE Guidelines are just that - guidelines. Hospitals are free to ignore them. They do not give you a right to anything. You CAN NOT demand to have an ELCS on the NHS.
What you DO have a right to, is the most appropriate care for your circumstances.
There is a subtle, but important difference here that affects the way in which you might frame things as there is a strong case that he two things might not be mutually exclusive. An ELCS might be the most appropriate care for your mental health if you felt traumatised by your first birth.
I have read a great many threads on the subject on MN. The disparity in care and access to ELCS across the country is shocking. Some hospitals will be fine and you'd have no problem getting one. Others, maybe one just down the road, will display a shocking attitude and effectively bully woman into a natural birth just through the hoops they make them jump through to get an ELCS. Women who may be emotionally already fragile. Its appalling.
Your request CAN be refused, but in practise this rarely ends up being the final outcome. If you are prepared to fight and keep fighting, I have only ever seen two cases where a woman hasn't secured an ELCS in all the years I've read threads like this on MN. The first, the woman went into labour early, and the other changed hospitals and did have an ELCS there.
The case you have to make is one about how being forced to have a natural birth would be damaging to your mental health. Your mental health IS a medical reason to have an ELCS contrary to much popular belief.
The NICE guidelines ARE an effective tool in supporting this, despite the fact they don't give you rights. They say that there is evidence to suggest that denying a woman an ELCS can be damaging. Which ultimately makes hospitals nervous as should they ever be challenged in a court, they might have a problem on their hands. Hence why some hospitals are going down the route they are, as they can discourage as many women as possible to do things their way whilst at the same time saying that they are merely ensuring that woman don't have 'unnecessary' surgery. Which is a matter of opinion give that the next hospital wouldn't have a problem with it and would deem it necessary for your mental health.
In terms of 'appropriate care' just saying you will have to make your case to a consultant is not really sufficient, though many hospitals have a woeful or none existant plan for the mental health of pregnant women, which has been highlighted in a report on the subject. If you are saying you are traumatised, they should be refering you for support in parallel with hoop jumping. If they are not doing this, then this gives you a good stick to beat them with. If the process of hoop jumping, is causing you stress and anxiety in its own right, use this as another stick to beat them with and say they are damaging your mental health with it.
I personally had an ELCS for anxiety for my first child which is unusual, but I found a good hospital (not my local), which I knew to be sympathetic to cases like mine and got referred through my GP. So I would encourage you to consider going through your GP or to look around at alternative hospitals nearby if that was possible for you, as a potential fall back, in case you are really struggling to get taken seriously or the process is causing you stress.
An ELCS isn't the magic solution for everyone. What I liked about my hospital was the fact that they had a good track record of providing support, which meant many women who requested an ELCS changed their mind of their own accord, without being bullied by hoop jumping. Hence why I stress the importance of 'appropriate care' being the support ad how it shouldn't be about an ELCS being 'bad' or to be discouraged - it should be about listening properly to woman and their fears and concerns.