Yes, when you use the mileage allowance method the actual expenses you incur to run your car (fuel, servicing, etc.) are not relevant to the business - you can think of them as “personal” rather than “business” costs, and when you pay for personal purchases using business funds as a sole trader, you account for it as a transfer to drawings, as you’ve effectively drawn money out of the business for personal use. The only thing you record in the business accounts as a deductible expense is the 45p/25p per mile for the number of business miles you have done.
So in your example the “expense” for the purposes of the business accounts is 45p per mile times 60 miles = £27, which you would log as a purchase against a dummy supplier as per the QuickFile knowledge base guide. Since there’s no money actually going out of the current account for this “expense” you mark it as paid from the proprietor drawings account - it’s not really a purchase in the usual sense, the point is that the mileage allowance you’ve recorded will be deducted from the net profit of the business, which reduces the amount of income tax and NI you have to pay at tax return time.
You can take the extra £7 that the business “owes” you if you like (the £27 mileage allowance minus the £20 you’ve already spent on fuel), tagging it as another transfer to drawings, but you don’t have to and it doesn’t affect the tax calculations whether you do or not.