Shared ownership was once only available through housing associations. Those who could not afford to purchase 100% outright would purchase anything between 25% and 75% of a property from a housing association and rent the rest. They would then have the option to buy further shares of the equity and ultimately the property outright as income and circumstances improved. The purchase of future shares would be based on the then value of the property (rather than its original price.)
In an attempt to stimulate the market after the crash the government got in on the act and introduced a range of options under the help to buy banner.
Help to buy shared ownership is detailed here https://www.helptobuy.gov.uk/shared-ownership/