No, the UK Visit Visa cannot be extended beyond its initial validity of 6 months. The standard practice does not allow extending a UK Visit Visa beyond the 6-month duration. The extension process is restricted to specific circumstances which include medical emergencies and compassionate grounds which are unexpected. Before the expiry date of your current visa you need to submit your application along with comprehensive supporting documents.
The UK Home Office maintains an expectation that visitors will depart the UK before their original visa period ends because such cases are infrequent. International individuals who plan to visit multiple times or stay longer should request a long-term multiple-entry visa instead of a standard visa.