You need to tread very carefully.

Unless you are able to demonstrate that magpies are killing songbirds etc. ON YOUR PROPERTY you could be at risk of prosecution.

And you need to be able to argue that other methods of scaring them off are ineffective.

I suggest you keep a record of the number of songbird nests, eggs, fledglings, visits to your bird table and feeders etc. and photograph e.g. eggs eaten etc. so you could demonstrate that the Magpies you shot were a problem on your property, and that your control measures have improved the survival rate of songbirds.

Its not enough to state that magpies are unpleasant creatures which destroy songbirds... so should be killed at every opportunity.

That is unreasonable, they are a native species and are expected to live in balance with other birds. Extermination would be no more acceptable than extermination of native raptors. Citation on an open license does not mean that they may be shot at will.

Just as it would be illegal to shoot a pigeon in your garden, unless you could argue that it was causing damage to your crops and that you had tried or could argue that other non lethal control measures had either been tried or would be ineffective. Or you could try the health risk option too

The bottom line is that shooting any birds in a domestic garden is just asking to be prosecuted. Its a different matter when engaging in genuine pest control over e.g. a smallholding or farm.