Cos,
Well, I think you may be looking for something that doesn't exist. In the UK there is no aerobatic qualification or license or anything like that. You can do anything you like within the limits of the airplane, the airspace, the ANO, and your abilities. No one signs you off to fly at a lower altitude (however you define that) but you do need to get a signoff from the British Aerobatic Association to fly in their competitions at anything higher than Beginner. But that sign off doesn't allow you to do anything other than participate in their competitions. Rule 5 always applies, unless it's specifically exempted like for an air display or a competition.
Your training and abilities notwithstanding, as long as you don't bust Rule 5, you can fly any way you like at any altitude you like, but it may not be smart to do so. For myself, I never practice aerobatics lower than a 1000 foot floor. That's because I competed in that range of airspace so I know what my airplane will do relative to that floor. Most often, though, I'm much higher than that, especially if trying something new.
You may be confusing aerobatics with a Display Authorisation (DA). A DA is given by the CAA after a pilot has passed an oral and flying exam and will be given with specific limitations, usually not flying aerobatics lower than 500 feet AGL for a new display pilot. for more advanced pilots, the clearances will be much lower, as low as 30 feet in some cases. A guy like Will Curtis probably has an Unlimited DA which will allow him to go down to 30 feet. This allows you to fly at whatever altitude your DA clears you for at official air displays in front of a crowd. It doesn't allow you to do anything else anywhere else, but if you want to, say, perform at the Goodwood festival, you would have to have a DA from the CAA. If Will Curtis wants to go perform aerobatics over his buddy's backyard BBQ, he can do that as long as he obeys Rule 5, but having a DA doesn't mean he can go do a display over his BBQ at 30 feet. Am I explaining this very well?
As for how I got to competition standard (a phrase that will have the likes of Sticknruda in stitches!), I took a couple of informal lessons with guys on some basics, had lots of long conversations on the ground with guys who really knew how to fly Pitts Specials, went up to about 6,000 feet and tried things out. I've always been very honest with myself about my own limitations (and when I wasn't, the judges sure as hell were!) and I only push them in small increments in order to learn. I'm at the point now where I wouldn't want to do too much else without some good formal instruction.
Hope that helps. If it was me spending the money, I'd ditch the nosewheel airplane and the AOPA thing, go find an instructor and tell him what you want to do, and just go fly to achieve whatever your goal is. But that's just my .02 worth. In any case, keep your enthusiasm up, have great fun, and fly safely!
Pitts2112