I've just looked on youtube and the standard method appears to be to run a knife along the edges, then peel off. This seems weird to me as glass weakens when scored and scores easily, plus there's no way to know whether the protective film is being properly cut and so the edge might fray and look messy.
The is a Wickes door btw. Is there any other method, perhaps applying heat to make the film wrinkle and come loose?