It's not ideal and I would never use a green / yellow wire for anything other than an earth connection. However, in the real world many do. Probably most heating system installations have at least one somewhere. But, it should at least have a brown sleeve over it (not insulation tape as it tends to come off over time) to indicate that it's being used as a live wire.
The same would apply to any blue wires used as a L. Although I do do this. You will find many examples of it being used for cables dropped to light switches where both wires are 'L' but one has brown insulation, the other blue.
The same would apply to any blue wires used as a L. Although I do do this. You will find many examples of it being used for cables dropped to light switches where both wires are 'L' but one has brown insulation, the other blue.
