Melly, your best defense in filing your claim is to take pictures of when your skin is good and when it is not. Make sure you are taking photos of joint areas (elbows, wrists, armpits, back of knees) as well as the palms of your hands and soles of your feet. Explain in writing what it feels like in terms of ability to move when your skin is bad as well as degree of pain. Let them know that the condition of your skin can be dependant on many factors such as overall health, weather, stress levels, contact with cleaning agents, etc. or anything else that affects your skin's health.
Remember, these people know NOTHING of what it's like to live with ichthyosis. They see your skin looking "good" or "bad" but to them it doesn't translate into how difficult it is for you to perform tasks such as getting dressed, cooking or cleaning, let alone getting yourself to work and performing a job. Compare right and left what it should be like for you to do things if you didn't have ichthyosis to what it is like because you do have icthyosis.
Those pictures are extremely important because the person reviewing your case file reads hundreds upon thousands of cases each year so they hear from a lot people who are in pain. You need to illustrate in as clear a manner as possible WHY you should not be working, not just that it hurts.
You and I and just about everyone on this board and anyone with a chronic illness knows that if you are in chronic pain, of course your mental health suffers. Things can feel overwhelming and it's really hard to cope. Tie that in... illustrate how on a day where your skin is bad how that affects you mentally. Does it make it harder for you to do normal everyday tasks such as vacuuming or doing dishes because you know it will hurt to be on your feet and using your hands? Again, these people have no idea of what it's like. They can imagine what it's like to have a broken limb but a skin disease is typically beyond their realm of experience.
I am female, and was born in 1972 with Lamellar Ichthyosis.