The expiry date shown on the visa is the date by which you should leave the UK or if your visa is in a category which enables you to extend your stay, we suggest you apply for further leave to remain at least three weeks before the expiry date. 

A visa in a Visit category cannot normally be extended.  But if your are unable to leave before your visa expires, for example, sudden death in the family, then you must apply to extend your stay on compassionate grounds.