Since you mentioned £, there's a good chance you're in the UK. The UK is something of an anomaly in the world in so much as you don't need to use CDFs because you can 'spread bet'. The principle is ultimately the same: you're making a bet that the price will change in your favour.
As others have said, this isn't investment and isn't a good idea if you don't know what you're doing. It's a possibly risky way into the field because your losses can exceed your deposit. It's generally pretty short-term, and so is highly susceptible to unpredictable temporary market fluctuation ('real' investing is usually longer term, and so based on the general trend of the market, which is generally less difficult to predict).
That said, half-way decent spread betting companies will check you out pretty thoroughly before you start, they'll offer a 'demo' account where you can trade with 'fake money' (ie. you make no deposit, and can make no withdrawals) until you're comfortable. Some do training courses and seminars too. When you first start trading for real, you'll need to put a 'stop loss' on every trade, and thus mostly avoid the problem of losing more than you staked (it's still possible to lose more than you staked with a stop loss, but in most cases your excess loss won't be ruinous, just eye-watering).
I worked for one such spread-betting company (a good, honest one at that). We once had an internal competition using demo accounts - the aim was to make as much money as you could in a two week period. I think we started with £10,000 each. A couple of people 'made' a decent looking amount of money in that time, but dozens more of us lost at least all of the money. It is possible to make money, but there's a far, far greater chance you'll lose all you're prepared to stake (and maybe more). Also, using a demo account is very different from using real money (no matter how much you tell yourself it isn't).