I didn't request a full H&S risk assessment, though nice to know people care
OK, distance wise, technically it would appear to be illegal because of the proximity of the road, theres the "….with the result a user of the highway is injured, interrupted or endangered" caveat though there's the question of what 'interruption' actually consists of. It doesn't appear that the law cares what direction the gun is pointing in within 50 ft exclusion, but standing any distance away from a road, or anything or anyone you don't own and firing towards it carries its own penalties, so I'd guess that the legislation counts equally when you are firing in any direction
Although the activity can't be seen, it can probably be heard, which given the unseen element might contribute to 'interruption' above (being able to hear nearby shots but not see where they are being fired too or from, would I some eyes be considered inconvenient)
The aspect i mentioned of 'making a gift' relates to both the gun and the ammunition, or rather the gun or the ammunition. So there is legislation relating the pellets on their own, but what the act of 'making a gift" is exactly, I'm not sure.