I'm extremely resentful towards this specialty in healthcare because every single dermatologist I've had my entire life have been so pompous, egotistical, and rude.
Switch dietician for consultants & boy can I relate to that. Unfortunately my experience has been that they don't like difficult cases & faced with it, they too often start to blame the patient & talk to you like an idiot & that's even at my age.
My DB was accused of alcoholism, I was accused of munchausen by proxy for chasing diagnosis & treatment for my then 12 yo who was gravely ill & written off to a wheelchair with an additional diagnosis that pointed to it being psychological & due to having an overbearing mother 
myself diagnosed with fibromyalgia & told it was a MH problem . I had to pay for private blood tests to show we actually had an advanced B12 deficiency as we have a genetic defect that means we can't process it properly which is serious if left untreated & needs B12 injections. Those injections got my daughter out of a wheelchair & walking again in under 2 weeks, but did any of us get any apologies for shitty treatment
Given your diet is so good I'd still see your GP & ask about testing for B12, folate & vitamin D as these are test a GP can run & might be relevant, plus everyone in the uK is deficient in vit D as it's the sunlight vitamin & we don't get enough sun. The others can be due to absorption problems which can have a host of causes, some of which such as celiac can cause problems absorbing all vitamins. So celiac screening would be a good idea too.
I'm not sure an nhs dietitian would be much help as they literally advise on diet. We saw one with my daughter & she gave no advice we didn't already know & then wrote in her report all of the things I'd told her as if she'd advised us of those things 🙄 complete waste of time bar her agreeing with chemical sensitivity causing eczema/urticaria
Thinking on it, do you eat or use creams with chemical additives?? Azo dyes & benzoate preservatives are the main culprits & in so many things, including the creams & lotions prescribed for skin conditions. So made things worse. So look at your face wash, moisturiser, any processed food, drinks etc for culprits. We found avoiding these cured my DDs eczema which was really misdiagnosed urticaria.
All her sensitivities are much improved since starting on B12 injections with Solgar high strength B vits & folate.
Are you hyper mobiles all?? Might seem an odd question but it can indicate a medical condition that is often missed & can affect absorption of food too. Look at "The Beighton Score" to see if you are hyper mobile. I had no clue I was as it wasn't extreme & is normal in my family
It must be soul destroying for you, but you will get there, looking at the bigger picture is the way to go though as dermatologist have ruled out it been your actual skin, so it's something internal. Your diet sounds excellent. So it's either your digestive tract not working properly or something alien you can't tolerate such as chemical additives
Some such as zinc you could try & self treat, others such as B12 needs testing as you might need injections if you can't absorb B12