As far as I am aware Cake Flour isn't available here in the UK. I have in the past just substituted plain flour and ended up with varying results. Therefore I wouldn't suggest doing that.
After a little research I have found the secret to making my own. You will need a US measuring (1) cup, a measuring tablespoon, plain flour, and corn flour (known as corn starch in the US).
Place two tablespoons of corn flour into your measuring cup.
Then fill up the cup with plain flour. I usually pour my flour into the cup because the bag is hard to get a cup in and out of without making a huge mess.
If you are of the dipping and leveling then do that with your flour, then take out 2 tablespoons and add in 2 tablespoons of corn flour. Which ever method is easiest for you.
I have a few cakes and bakes to make that are American that require Cake Flour so I made 2 cups to start with. It's so easy that you can make it as you need it or make it in advance and store it in the cupboard!