I'm from the States so I'm not sure how to best advise you to move forward, but I would say that you shouldn't give up. There are likely other ways to get seen and evaluated, as I doubt they're going to stop diagnosing ADHD in the UK completely. Maybe you need to see a specialist instead of going through your GP. Also, testing isn't necessary to get an ADHD diagnosis but sometimes doctors feel like they need it in order to do so. You may want to talk with your GP directly and inform them of what happened. My guess is that it would be best to seek out a therapist or psychiatrist to get them to help with diagnosis. If that's an option. Again, I'm not sure how the system works across the pond.