Usually for magpies, a Larsen trap is far more effective, but then you need a call bird to start off with.
Before even committing to attempt a shot on your quarry, you need to be repeatedly able to manage 5p-or-less sized groups at your intended range. If you are not confident at all, keep shooting paper targets until you are.
Magpies come under the general license as a pest, so are freely shootable. You must, however, take proper precautions when shooting on your own property - like a backstop. Have you a plan if the worst should happen and you only wing a bird? Having the utmost respect for your quarry is the first and hardest rule of shooting - a quick clean dispatch is always best, but things are not always 100% repeatable in the real world, and you may have to find the bird and finish it off yourself.
Some good shooting forums out there - www.pigeonwatch.co.uk is probably the friendliest, and has a dedicated airgunners area. Google "airgunbbs" too - lots of info there (sorry no link as its blocked from work). You may also want to think about insurance too - I've used BASC for decades, and needn't be expensive if you go "airgun only".
RT - please note that if that 22lb spring was producing 22ftlb muzzle energy, it is classed as firearms and will / would have needed a Firearms certificate (FAC). The lack of which when using such a powered weapon can earn you a swift holiday at Her Majesties Pleasure.
I can always pop over with the semi-auto shotguns