To get a visa as a unmarried partner you have to prove that you have been in a relationship like a marriage for at least 2 years - the checklist gives you examples of the type of evidence you need.
"Evidence of previous cohabitation. We would suggest the following examples:
Joint commitments ( such as joint bank accounts, investments, rent agreements, mortgage, etc.); or If there are children of the relationship, a record of their birth entry; or Correspondence which links you and your sponsor to the same address; or Any official records of you sharing the same address (e.g. Doctor’s record, etc.); or Any other evidence that adequately demonstrates your commitment to each other"
If you can't provide enough of this evidence over at least the last 2 years you are unlikely to be granted a visa as a unmarried partner.
