Aware that using the GUI is very beginner esque, but I need to be able to type before I can get to grips with Ubuntu!
Click on settings, keyboard, and then, in the bottom left, Layout Settings. On the left there is a panel that displays currently installed keyboard layouts. By default, only English (US) is available. Below the panel, click the + icon and select English (UK). This will add English (UK) to the panel. Use the ^ symbol to bump English (UK) above English (US). The panel is hierarchical, so now English (UK) takes the priority.
You can even select English (US) and use the - to delete the keyboard from the panel.