You can change your mind right up until the point you exchange contracts. The buyers of your house probably won't be very impressed but that's life when buying a house. Chains frequently collapse in this way.
The same happens when Surveys etc are done - if anything is wrong with the property, you get the option to change your mind. I pulled out of a purchase when i received a survey saying that it was over valued and loads of work needed doing. I lost the �500 the survey had cost me but saved a heck of a lot in the long run!