A UK entry clearance gives you 'formal' permission to arrive/depart any time between its start and end dates.
The form asked for your travel dates so that they can try to align the visa's start date so that your time in the UK is maximized. Once the entry clearance has been issued, it's ok to change your actual travel date.
You can arrive at 12:01 AM on the start date of your visa or any time after that over the visa's life time. Trying to arrive earlier than the start date is not allowed.
And remember, you will still have a landing interview, and if your actual dates somehow contradict the premise of your visit, the IO may become worried. Also remember that you signed an agreement to advise the British issuing post of any significant changes in your circumstances. It's down to you to determine if your change of plans is derived from a reportable event.