System Preferences > Keyboard > Text. Click the + symbol to add a keyboard shortcut. Click in the left empty Replace text window and place GBP there. Tab, and enter option+# (£) in the With text input area. Click outside of this row to set the contents. Exit System Preferences.
In the application where you wish to have this substitution performed for you, in the Edit menu > Substitutions submenu, select Text Replacement. Then when you type GBP, a floating panel will open below GBP with the pound sterling symbol — just press the spacebar.
Another approach is to use the special character viewer from the bottom of the Edit menu. If ISO-8859-1 is not one of your categories, use the gear icon in the top left to add it. Click on the £ symbol, and in the enlarged view on the right, click on the Add to Favorites. In the future, when you want to add the pound sterling symbol, launch this character viewer, click on Favorites at the top of the category column, and then double-click the £ symbol to have it inserted at the current pointer location in your document. Not as efficient as typing GBP.