Buying a house alone/together?

Right i am in the process of buying my second house, i have been with my fiancee for 3 years. Now this house is coming from my wages and savingsand the house we are selling has all been out of my pocket apart from 3k max over the 2 years we lived there. My girlfriend wants to be on the deeds of the new house but i am afraid of being ripped off, its happened before to me you see, i have said i will add her when we get married but she wants to be on them when we exchange contracts, is there anything i can do to put her on the deeds but make it so she isnt entitled to half as she wont be contributing financially, i wouldnt mind if it was only 100k or so but its a £350K house so i have a lot to lose.