Lets take the example of a long trade on ANTO. Current price is 558. Lets say I want to go long at 10 pounds a point with a stop at 510. If the trade goes against me I can stand to lose 480 (in fact if it gaps I could lose more but that's a different discussion and you can have guaranteed stops to address this). IGindex is current stating that you will need a deposit of 814 pounds in your account to make this trade.
If I was to buy the shares directly this would mean I would have to pay 558 x 10 = 5580 (plus commission slippage and tax). So the outlay is less with SB. However that should not be your main concern. Which ever way you decide to trade you should limit your risk to 1-2% of your account. So in the example above you should not make the trade unless you have an account of around 24K.
For your last point you are correct, you never own the shares.