An option is an agreement that gives you and only you the right to buy the property in a specific timeframe e.g. 3-6 months. Make the agreement legally binding, and then sell the Option for profit.
For example: You agree to purchase a house for £75,000, when its true surveyed market value is £100,000. The seller signs an option agreement which gives you the exclusive right to buy the property for £75,000 within the next 3-6 months. You have to pay the seller for the agreement to make the contract binding but the amount can be as little as £1 if both parties agree. After the agreement is signed you register the option with the land registry. At which point you can sell the agreement for £10,000 or as much as you can get.
In this situation you need to have a lot of pegs in the right holes. The seller is happy because they've sold the house. Your buyer is happy because they've got a below market value deal and you’re happy because you've just made £10k or more for relatively little work without owning the property.
The following examples show the various types of mortgage used to purchase an investment property.