The Border Force Officer can only give you 6 months or nothing. As it's a once-only leave, it expires the moment you depart from UK. But you are still bound by your original request to stay for 6 months, and you are expected to leave for home as per your original plan. It would make life easier if they had the choice of granting shorter leave, such as two weeks, one month, two months etc based on individual situation, but it's an option nor open to them. So they have to stamp you in for further 6 months, but you aren't suppose to misuse it and just prolong your stay in UK. This will get you into trouble down the line.