I see you have two very closely related questions. The first being what counts as exiting the UK, the second being, how often can you enter the UK with a multiple entry visa?
The answer to the first question is quite simple. You have to leave the country through an immigration check. This is as simple as crossing the Channel by boat. You can also fly to any other country. It does not have to be your home country.
The second question is slightly more complicated. For more information about this, see Doc's detailed answer over here: http://travel.stackexchange.com/a/18947/36405https://travel.stackexchange.com/a/18947/36405. A quick summary: A visa is issued for a specific purpose, like vacation, visiting family/friends or business. It is not for living in the country. So-called visa-runs are certainly possible (going to France and immediately going back), it is against the purpose of the visa. Use your visa for it's intended purpose.