Strictly speaking, you require one for an MOT, whether it was originally fitted or not. In practice, you can usually get away without it.
The regs say that you must have some way of washing the window, but they do not specify exactly what that involves. I have a squeezy bottle of water in my glovebox labelled 'screenwash'. Since I can demonstrate that the windscreen can be washed with this by reaching out of the driver's side window (or the sunroof), it's a pass!