There was a lad in the UAS who was an engineer joining up on a bursary even had his civi PPL and was a rising star in the gliding world. Even though he was perfectly legal to fly the machines solo on his PPL he could never go solo with the UAS.
How long ago was that?
I used to be in the UAS and I know that recently they have changed their medical standards so that they are roughly equivalent to what an NPPL needs to meet (which I believe is similar to what an HGV driver needs meet as well). There is no longer a formal RAF medical as such, all you have to do is get a form signed by a GP declaring that you don't have an conditions that would render you unfit to fly. I don't know about migraines but I would be surprised if they prevented you from flying in the UAS.