When you were in London, you were NOT at work.

Are you exposed to some organic solvents like mineral spirits, or
xylol, or acetone (cleaners) at work? Or some kind of "dust" from working with tools?

Have you tried to cut back on washing your hair?
Do you blow dry afterwards? Try changing your routine. Say, drop the brushing (this pulls on the hair follicles). There are combs with very wide teeth out there to reduce pulling the hair.
https://www.google.com/search?q=wide...iw=981&bih=662

I looked up severe scalp pain, and found one medical answer that is temporal arteritis or vasculitis. This is an autoimmune disease where the blood vessels supplying the scalp become inflamed.
Hemoglobin may be low, serum alkaline phosphatase is increased, sedimentation rate and C reactive protein are raised in this illness. Were you ever evaluated with tests for this?
Most Dermatologists and neurologists don't deal with this type of problem. Your regular doctor can test you for this possibility.

Also a fungus can be a culprit. Long term use of steroids can result in a scalp fungus. Nizoral typically is used for that, in a shampoo. Excessive washing of the hair and blow drying will enhance fungus if it is there.
This link explains severe scalp fungus may cause pain:
http://www.mayoclinic.com/health/rin...=complications

This is a general article on the topic of scalp pain.
http://www.wisegeek.com/what-are-the...scalp-pain.htm

I do not believe your issues are psychosomatic. So don't worry about that.

I'll give you this little story...to illustrate how a small thing can cause massive pain!. Long ago when nitroglycerin was only used in bombs, and dynamite, the factory workers who made it developed a mysterious illness. They would work M-F, and be off Sat and Sun. The next Monday they would return to work and within a short period of time would develop blindingly severe headaches. After much investigation, it was found that their hats they wore absorbed the nitro, into the bands. Then the workers would absorb from the hatbands, the nitro into their bodies. They would develop a tolerance to this during the week, (nitroglycerin causes a rapid tolerance and that is why it is not used much anymore as a chronic medicine for the heart). Over the weekend their bodies removed the drug, and Monday it would start all over again. This led to nitro being used as a cardiac and vasodilator for many decades. Today is is mostly used for angina attacks and daily use is much less common because of the tolerance factor. But this story illustrates how an invisible factor in the environment can work to cause tremendous pain and grief.
Nitroglycerine can cause severe headaches, but they go away as the tolerance develops.

So I do believe there is something in your environment triggering your symptoms. You just have to be patient to find that answer.