Welcome to our SCLS community, Kevin!
I hope for your sake that some diagnosis or another is confirmed soon, so at least you and your doctors will know how to proceed. In the meantime, please direct them to this site and particularly to the "Disorder Details" and "Disorder Resources" sections, so they may catch up with the state of knowledge. (You can also print out those pages and hand them over, demanding that they become part of your medical records.)
And if it's any consolation, SCLS in the U.K. has been reported previously in the medical literature, and we even have another patient in this community who hails from the U.K., see _SystemicLisa._
As to your question, several years ago, when I was taking the same medications as you are now, I did not have your experience. The gabapentin worked just fine to lessen the pain in my hands. (I had lost all feelings in my feet, so they were not a source of pain.)
And if your diagnosis is confirmed and you cannot tolerate high doses of terbutaline and uniphyllin, or they fail to prevent a future episode, then your doctors should put you on a therapy of monthly IVIG.
Good luck to you!