I want to have my headlights/rearlights tinted, is this allowed?

No, it is not permitted to apply film to lights both front and rear. However, provided that it is done properly and not too dark, the MOT inspector will approve it 9 out of 10 times because it is not visible that it is a film.
However, we would like to emphasise that having your headlights/rearlights tinted is at your own risk.
en_GB