Right'o, I plugged my Mac keyboard to my PC...
Don't use the upper shared row to type the numbers. Use the numerical keyboard to the right. Also, the Right Alt is not the Right one. The Left is. (Hahaha! *cough cough*)
Left Alt+0163 should produce a £.
Also, on my Apple keyboard the £ is in the same place as on my Logitech keyboard (thought there's no actual £ label on the Apple keyboard itself). Ie: Right Alt+3 on Mac keyboard, Right Alt (aka Alt Gr)+3 on my Logitech keyboard.
/This post typed on a Apple Keyboard in a Windows OS