Your St Peters kit will produce a beer of just under 4% ABV if brewed to 23 litres which includes a small allowance for the alcohol produced at the carbonation stage.
If you want to increase the alcohol content, simply add more fermentable sugars, or brew short (i.e. less volume)
So, for example, for every 100g of cane sugar you add, you increase the ABV by just over 0.2%.
But if you added no extra sugar and reduced the brew volume to 20 litres you would produce a beer of about 4.5% including the priming sugar allowance, with a corresponding pro rata increase in bitterness and flavour.
If you want to play about with additions and volumes I suggest you use the calculator below using 'Extract' as your brew basis.
https://www.brewersfriend.com/homebrew/recipe/calculator