Also could be argued that just because some obscure pellet, only found in the USA, can take many a UK rifle go above the limit means most UK air rifles are running illegally.
I think any Court would take a view on normal expected everyday pellets, not extreme or special. Why I said resent pellet designs have muddied the water. Exotic pellets are becoming more normal.
People don't have to test their air rifles if they don't want to, nor does the law demand they do. How many £14 tins would be expected to be tested? Ten, Twenty? How much testing equipment should come with every gun sold?
People don't have an MOT Test Centre on their driveway.
If you are an enthusiast, or tuner, then most likely you will have all the kit. For Joe Average they don't require any of it.
The main point is:"If you know". Well, that is intentional and bad, unless you do something about it rapido. Joe Average doesn't know, nor going to make the investment to find out, and importantly shouldn't have to.