well, every litre of water the OP pumps to his loft is one less litre that the water company has to pump to his loft. I doubt that it is significantly less efficient to pump the water oneself, and therefore the environmental costs would be small. If one lives at a relatively high elevation then it would be significantly more efficient to pump oneself.
There are significant envionmental benefits from harvesting rainwater.
Houses represent an impermeable covering to the ground, causing increased runoff and leading to higher peaked, flashier(1) flood flows in rivers. This causes damage both to the natural and built environment. Harvesting mitigates, to some extent, the increased runoff.
During dry periods, the use of stored rainwater reduces the demand for water abstraction from rivers or aquifer and helps to mitigate the environmental damage caused by low river flows and lowered water tables.
There is a school of thought which says that rainwater harvesting provision should be madatory for all new housing.
(1) the faster the flow rises and falls again, the flashier the flood river is said to be.
Dave