You will be deemed to be giving away half of your interest at Market Value, and therefore a Capital Gain will arise for you. Your fiancee would be deemed to acquire her half share at the market value. Depending on the increase in value, the reliefs available, and the availability of the annual Capital Gains Tax exemption, this may or may not give rise to a tax liablilty.
If you wait until you are married, you can give her half the property with no Capital Gains Tax implications.