No you are not able to marry on a visitor visa

http://www.ukba.homeoffice.gov.uk/vi...s/eligibility/



You must also show that, during your visit, you do not intend to:
  • take employment, produce goods or provide services, including the selling of goods or services directo to members of the public;
  • undertake a course of study;
  • marry or form a civil partnership, or give notice of marriage or civil partnership;
  • carry out the activities of a business visitor, a sports visitor or an entertainer visitor; or
  • receive private medical treatment.