There are many initatives in London that are designed to help you onto the property ladder. The First Steps service is a website that draws many of these together in one place. You need to register with them and you can then view all the different properties in London that are more affordable.
New Build Homebuy is probably the best known of these schemes and it allows you to buy a share in your new home while leaving a share with a housing association. You would then typically pay your mortgage each month for the share of your home that you own and rent each month for the share that the housing assocaition own. The mortgage and rent combined means that you can pay less than you would with a normal mortgage, but you also own part of the property. Most schemes enable you to increase your share in the property when you can afford to.