I had my visa start on the day they approved it, as the entry clearance officer asked me when it should start, since I wasnt sure when I was leaving (they were saying it may not have been a same-day application, but it was
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). However if you have the tickets bought and know thats the day you are leaving they will generally make it start that day, I believe you can have about 3 months or so leeway...not 100% on that last bit though.
In my case its only, I think one weeks difference, and we are getting married in 3 months time anyway. Ive been here a bit over 2 weeks so well within the visa time frame.