We would recommend the plan that protects 3 devices over a year (currently reduced to £34.99 from £49.99), which would effectively allow you to keep your PC (or Mac), phone and a tablet safe.
You can however find the same plan for cheaper elsewhere. Amazon has the 3 devices 1 year plan for £21 if you choose to receive the activation code by email. Cheaper yet, MyMemory is offering the same plan for £19.99.
MyMemory has discounts on other 1-year plans as well, with protection for 5 devices for £22.99 (compared to £41.99 from Kaspersky directly) and 10 devices at £27.99 (unavailable from Kaspersky directly).