Sole Trader - allowable expenses

Wonder if anyone can assist in an area that is confusing me a bit. I look at the QF tax summary and can see that sales less cost of sales minus expenses = taxable profit. The taxable profit figure can, potentially, be further reduced through ‘deductions’. Am I correct in saying that allowable expenses (e.g travel costs, office costs, advertising/marketing costs etc) would fall into this deductions category?

If that is the case, I have a query specific to my business activity. I currently use my own car for daily business trips and note the miles done very accurately. Looking through the website guide notes, I have decided to use the simplified expenses option to work out costs (where you multiply business miles by a fixed rate per mile). My financial year is in line with the tax year and this tax year to date my business has NOT paid for any fuel or indeed other running costs associated with the car. (all done thus far from my private monies). I know the monthly costs so can I put these through as ‘deductions’ that will show in the tax summary, and if so, how would I do it?