Is it legal to sell your collected rainwater in England?

I believe that if you were selling enough to be profitable you would run into trouble. But selling any amount would probably require licensing, so that alone would make it impractical, but I'm no legal expert.

With rules and regulations like this, the issue never stems from the act of collecting rainwater, or even selling it. The concerns of the authorities are more to do with groundwater contamination if you're not doing it right. I believe this would be the only reason any authority would ever have an issue with you taking on a project like this. Safety is their focus.

This. Can you imagine if someone had collected a lovely batch of e-coli in their water drums, sold it and infected half a town with the illness? Questions would rightly be raised about why the state hadn't done more to regulate the sale and monitor the safety of the water - even if it was brownwater. If water is seriously infected with something, all it takes is a few droplets to enter your system while you water your plants and you'll be struck down with the bug.

This. Can you imagine if someone had collected a lovely batch of e-coli in their water drums, sold it and infected half a town with the illness? Questions would rightly be raised about why the state hadn't done more to regulate the sale and monitor the safety of the water - even if it was brownwater. If water is seriously infected with something, all it takes is a few droplets to enter your system while you water your plants and you'll be struck down with the bug.

This, again. I would be constantly worried that the water I sold would cause issues.