We do need some further information to advise properly, but this reply is only for informational purposes. For the tier 1 investor visa, it is for one person to apply as the main applicant, but if you have a partner who you have lived with for two years or more, he or she could be your dependent in the application. You will need to prove the two years of co-habitation, too. If you broke up, the dependent's visa would become invalidated.
How can I invest in the U.K. tier 1 investor visa jointly with my girlfriend?
My girlfriend and I were told that even if each one of us does not have enough funds required by the U.K. tier 1 investor visa, we could invest jointly since we are partners, even if we are not married yet. Is this true? How do we prove our relationship? In our case, who would be the principal applicant and who would be the beneficiary? What could happen to our case if we broke up and one of us decided to drop the application?
If you have lived together for at least two years then she can come to the U.K. as your "unmarried partner." As long as you can show you are in a relationship "akin to marriage," then she can enter the U.K. as your visa dependent just as if she were your wife. In this scenario, if your relationship were to break down then she would have to leave the U.K. You cannot both be the main applicant in this scenario; you cannot share the money as a "team." If you were entering the U.K. under the tier 1 entrepreneur scheme (where you would need to invest £200,000 in the U.K.), then you could share the money and both be the main applicants. This scenario is available for any entrepreneur who wants to share £200,000 and enter the U.K. to invest in the same business; you would effectively be business partners.
You can certainly use a joint fund to make the application. You must prove you have lived together in a genuine relationship for at least two years. You must remain in the relationship to retain the visa. The documentation to prove the relationship can be discussed when dealing with an adviser.