The most likely thing the immigration officer will be looking for is whether or not you can support yourself for the period you're in the country - particularly given your age. They will be keen to establish you're not going to attempt to work whilst you're here. In this case, being able to show your period of stay is sensible (you haven't told us how long it is, which might well influence what the officer looks for in admissibility terms), and you should be prepared to produce your return ticket and be able to show you have a good reason to return to Canada (a job there, for example).

You'll almost certainly need to show evidence that you are staying with friends given the cost of hotel rooms in the UK isn't generally within the reach of teenagers travelling alone, and don't be entirely surprised if you're selected for further questioning whilst they confirm that - yours will not be as straightforward a case as most. If that happens, just tell the truth, let them make the phone calls to your friends who'll verify your story, and you'll be fine if slightly delayed.

It's actually difficult to know if your August entry will help you or not - it might show that you met the criteria then and so make this entry more straightforward if your financial circumstances haven't changed, it might raise a flag because they'll wonder why someone your age is coming back so quickly (which may raise suspicions you're returning to work). No-one can really predict what thoughts will go through an immigration officer's mind ...