Things have changed in recent years and adult ADHD is now officially recognised as a separate condition. All the major ADHD meds - Ritalin, Vyvanse, and Dexedrine - are scripted for it.
If you have ADHD as a kid you need to be assessed again as an adult to see if you still have symptoms since adult ADHD is seen as a separate condition, but as long as you still have symptoms they can keep scripting you the same meds.
The main bottleneck is the lack of resources given to it. As you can imagine adult ADHD is not a priority for the NHS. Adding to that, you need to see not just any psychiatrist, but one who is an ADHD specialist. Waiting lists for psychiatrists are long enough already so this adds up to waiting lists of up to 5 years for adult ADHD and this is why dex remains such a rare script.
So yeah I got my dex script from a private psychiatrist to skip the waiting times, but then I got it transferred over to my GP on the NHS, which was a smooth process.
I helped a few of my mates do the same using online psychiatrists (much cheaper than normal private psychs), they now have the same script. The NHS has tightened things up a bit when you transfer it over, so the GP usually has to bring it to some committee to cover their arse before accepting the script, but usually they will let it through especially if it's Vyvanse (Elvanse is actually the trade name in England, or Tyvanse in Ireland). The NHS likes Elvanse because of the time release that you can't beat without doing some actual chemistry since it's a prodrug not just a normal slow release pill. Dex is harder to get but you can get it, you just need to try the time release pills first.
I have ADD a few drs have diagnosed me with it. They refuse to give me adderal which does do shit to me anyway. They put me on Stratera which, same thing, NO CHANGE! 
