You don't need to prove that you will marry to get a fiance visa, as that is often not possible, but you do need to have the intention. As has been said, at the moment you can apply for an unmarried partner visa if you can prove that you have been living together for two years. Time spent on visit visas can count towards this.
You really need to have a good search through the UK VVisas website to see what other categories fit. If you are a student or recently graduated, perhaps a BUNAC would be suitable.
Victoria