Hey there! It's great that you're thinking this far in advance.. I wish I had done so

If you're certain you'll be sponsoring her in the future, than I would advice just to casually start collecting all the relationship evidence that you'll eventually need for your application - for example, go out of the way to take photos of you two with friends, family, etc, as well as casual photos (for example, wearing pj's so it looks like you're comfortable with each other!). Save your call histories and texts, as you can use those as a great example of your relationship.
If you can move there, or she can move here, that would be great to prove a common-law relationship where you live together for a year non-stop (but you can both go on vacations together, for example). Just one thing to note is if you do apply as common-law, you'll need to have been together for at least a year when you submit the application. You can't submit the common-law app after you've moved apart from each other!
One thing to note is you will not be able to apply as conjugal partners - that is only for people who are
unable to get married or be together based on immigration laws between countries/religious reasons. Being from England, she is able to come visit you, or apply for a work visa, as are you to go to England. I believe your best bet is to continue your relationship as it's going, and make sure to keep records as proof. It doesn't have to be anything serious now like getting a house lease together or sharing a bank account, but if you can prove you've had a long distance relationship from now until the date you apply (whenever that may be), it'll really help your case.
When you do apply, you can either apply as common-law like I've stated before, or you can get married and apply as spouses. Either way, the processing time will be the same, but you'll need more 'official' proof to sponsor as common-law, such as joint bank accounts, life insurance/benefits, shared utility bills, etc. You will
have to provide strong evidence that you've lived together for the year prior to applying - such as mail in both of your names to the address you provided for each month you lived together, a shared lease agreement, a notarized letter from your land lord, and letters from friends and family.
Hope that helps and good luck!