If you are seeking to come to the UK to marry your fiancé(e) there is a special type of visitor visa that you must obtain.
You will have to provide evidence to the Entry Clearance Officer that you are seeking to enter the UK for the purpose of marriage and you will have to provide information about your partner in the UK.
Once granted this visa will allow you to enter the UK for a period of 6 months. After the marriage, you must return to your Country of Origin before your fiancé(e) visa expires and from then you can make an application for your UK Spouse visa.