Hey, sorry I missed your reply before. Who'd have guessed I wasn't paying attention? :p
Anyhow, I was really worried, as well. What I experienced wasn't so much flat-out resistance, as abysmal ignorance. I think the general understanding of ADHD is a good 10-15 years behind current research. For adult ADHD, it can be even worse, because there's still a strong belief that ADHD is something that people just 'grow out of,' as if you wake up the day after you leave high school, to find your brain's been magically rewired. I had to do a lot of patient, calm explaining and educating, and because it required a referral to a consultant, and then back-and-forth communication between the consultant and GP, it took a while.
Unfortunately, how your new GP, or a psychiatrist, responds is a matter of luck. Which isn't really any different than the U.S.; some doctors are brilliant, some suck. Personally, if you find one that sucks, don't get lulled into thinking 'well, it's the NHS, I'm stuck with it.' Find another doctor, even if you have to travel further. Write to PAL (patient advocates) and the heath board, and complain. If you're not getting anywhere, write to your MP!
The NICE guidelines are clear; this is a real thing, that they need to address, no matter what age you are. And since you've already received treatment for it, as an adult, they can't just pretend you don't have it. You'll be fine, eventually. Just do all the research you can, and be prepared to explain the facts.