It depends on your hardware, your operating system and the application you are using.
Some notebook PCs have a way of simulating a numeric keypad by using additional meta keys (Fn, Ctrl, Alt etc)
Some operating systems support alternative keyboard layouts or input method editors. For example I have a Windows PC with a UK keyboard (and hence no problem with £) in the system notification area I have a keyboard icon from which I can choose UK or UK-extended keyboards. The extended one lets me compose additional accented characters. Maybe there is an extended keyboard for your locale that supports a key combination for £.
Finally applications such as MS Word have their own means of inputting special characters - see the application help.