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.

There is no guarantee of success. But here are a few things you can do to maximize your chances:

• Put your marshal to increase your levy (the middle button). This will not only increase the amount of levy you get but also improve the rate at which losses are replenished. Choose the county with the most levies if invasion is imminent or in the county with the most depleted levies if you have 6+ months.
• Choose your battles. Never fight a battle you can't win. If they are after Scotland they will want to occupy some land there. Lucky for you, Scotland is full of mountains. Find generals with rough terrain modifiers and high martial skill and try to get yourself in a situation where you can defend in mountainous terrain (see also below for tips on how you might achieve this). This might not always be possible but if you can arrange yourself to be a defender, it can drastically improve your kill ratio.
• If you have a bit of money, hire some mercenaries. Don't forget you can ask the Pope for money and borrow 300 gold from Jewish moneylenders (best in that order, as borrowing reduces church opinion). There are usually some smaller mercenary bands that hire for ~150G with ~25-50G/month. These still typically have 4-5k troops, which is often enough to tip the scales. Your goal should be to hire them just long enough for them to get near to 100% morale and get into a battle; if you win a few battles, you'll deplete the enemy army enough to be able to dismiss the mercenaries and maintain a troop advantage.
  • For this reason, mercenary armies serve as the perfect bait – if the enemy won't engage in an even battle, leave a small army by themselves in a defensive county, wait for the enemy to engage and drop your main force in by from a nearby county. Make sure the travel distance between your two armies is small, or you risk losing the battle before you can reinforce.
• Use ships. Ships move really fast compared to armies, especially if the armies have to move through mountainous terrain. When you see an AI trying to get to a province by foot you can therefore arrange to pretty much always have your soldiers waiting for them in a nice mountainous province. At the very least it will deter them from progressing. Also, boarding a ship is generally faster than fleeing over land, which can help with never losing a battle.
• With ships you can also try to catch the enemy before they group armies into bigger stacks. Sail around your coast and place all your troops in the same sea province into ships, this way you can group before they do. Then deliver the troops to their unsuspecting small armies.
• Be aware of the morale penalty from landing from a ship: armies restart at 50% morale when going from sea to land. There is also an offense value penalty when going from a ship straight into a battle. These can combine to turn a battle into a defender's victory if the attackers don't outnumber by roughly 2:1.
• In combination with deterring your enemy from progressing, it is very nice if you can keep them stuck in a province with low supply level. Just wait for their numbers to drop and then jump them with your troops which are nicely loaded on ships and can get in almost at will.
• If you're fighting a defensive battle, see if you can draw it out through a winter. This can drastically lower the supply levels in a county and force the enemy into taking attrition losses, abandoning sieges and generally drawing out the war, which will gradually grant warscore to the defender if you keep from losing too many sieges.
• Finally, if all else fails, note that AI is very fond of assaulting. This will deplete his troops and hopefully allow you to defeat his army. Once the army is defeated go in and retake the provinces.

Like I said, none of this is guaranteed to work. The AI might perform better than usual and not allow you to take advantage of various game mechanics I mentioned above, or the terrain and troop placement might not allow you to do so. But hopefully it can increase your chances.