There are broadly two types of lung defect which can be picked up on lung function tests:
1. Restrictive defect, caused by decreased elasticity within the lung tissue or the chest wall. This will result in a normal shaped curve (time on x axis and volume of air expelled on y axis), but one of much smaller magnitude than normal, in other words a decreased total lung capacity. Since the shape is normal, this implies the rate of expulsion of air from the lungs is normal and so the peak flow will tend to be normal.
2. Obstructive defect, caused by the airways collapsing as you try to breathe out, as may occur in chronic severe asthma or chronic obstructive airways disease. Here, the shape of the curve is flattened, although the total lung capacity may be near normal, or even raised, as air is trapped in the lungs and can't be expelled. Here, the peak flow will be low.
What you describe fits into neither of these patterns. You can't have obstructive airways disease and have a normal peak flow. Similarly, you can't have a restrictive defect and have a normal lung capacity.
Why don't you go and see your GP and see if he can figure it out? If he can't, why don't you get a private referral to see a chest physician. It will probably cost you about £150 and he / she will give you a definitive answer. I strongly suspect there's nothing wrong and you're just rubbish at doing the test and need more practice.
If there is something so wrong with your lung function tests, you have worries above and beyond not getting a class 1 medical.
QDM