I entered in October for two months. Two months is what I honestly expected to stay. Later, we did decide to extend my visit, but that was never our intentions when I entered the country. My suggestion would be to book a longer visit since you said you want to stay longer than the initial 2 weeks. Things DO come up that prevent people from leaving right away (in my case, it was illness and death in Tim's family), but it's really bad to enter the UK and lie to the I/O. Plus, if the I/O thinks you're being deceptive, you could wind up with a stamp for the exact length of time your tickets say, and then you won't be able to change things.
Like above posters said, if you bring in more clothing than is expected for a 2-week visit, it really could throw off the I/O. Granted, you don't have your suitcase with you when you go through, but if you get detained, they might search your luggage.