Reforming the Roman Empire as Catholic

According to a loading screen message, right click on the message, which will show you a window that lets you customize this exact kind of message, without looking for the right kind of message in the long list:

Click on the coat of arms of the kingdom of England title. You will see a check box where it says de jure, check that. Then, at the top right of the box, you will see a cost of arms for the de jure liege title of the Kingdom of England, which should be the British Empire. Click on its coat of arms. Then hover over the create button. This will tell you exactly which conditions you need to fulfill to create the title.

This button is also how you create the title and become Emperor. It will be greyed out until you meet all the conditions in the hover-over tooltip.

Here is a three-step illustration of how to find the Empire of Britannia by using the de jure checkbox and liege coats of arms in the Realm view:

Alternatively, you can always use the Find Title button below your world map. Click that, type part of the title you're looking for, and then it will show you matching titles in the dialogue box. Clicking a title will open the same view:

The best way to learn how to use the more obscure parts of the game's interface is by playing the in-game Tutorial. It explains everything and is worth doing once.