Section 1 (1) (a) makes it an offence for any person to intentionally kill, injure or take any wild bird.
Section 1 (1) (b) makes it an offence for any person to intentionally damage or destroy the nest of any wild bird while it is being built.
Section 1 (1) (c) states that it is an offence for any person to take or destroy an egg of any wild bird. This therefore gives every wild bird in Britain, whether it is a nesting bird or visiting species, full protection.
Under Section 2 it is an offence for any person to have in his possession or control any live or dead wild bird or any part thereof: or any egg or part of an egg of any wild bird.
There are certain exclusions to these rules which apply to problem birds such as woodpigeons, crows, certain species of gull and corvids, but none apply to birds of prey.
Birds of prey, including owls, are further protected by making it an offence to intentionally or recklessly disturb them while they are building , or in, on, or near, a nest containing eggs or young, or to disturb their dependent young. This can be committed by bird watchers or bird photographers as well as by egg or chick thieves.
(Above information is taken from leaflet by the Partnership For Action Against Wildlife Crime)
In short, all owls are protected by law in the UK, and in the case of the barn owl, which has additional Schedule I protection, you can be heavily fined for disturbing them at the nest site either physically or by photographing them, without a licence.