My girlfriend and I both have Ukrainian citizenship. We're planning to visit the UK at the end of April. While filling in the application form for visitor visa we specified our relationship status as unmarried partners since we have been living together for more than 3 years and I'm going to be her sponsor. Closer to visa appointment I started to google some information, like what documents we should bring to prove our relationship etc. All the information that pops out is about the case when a person already settled in the UK wants to bring his partner. Did we fill in the form right?
If you are both just visiting the UK, the authorities will not be very bothered by what your relationship to each other is. They are really only concerned about whether you will both leave at the end. Unless there is something that doesn't make sense about your story they won't care if you are legally married or how long you have been together. That is probably why you are not finding much information about it.
It might be helpful to have photos of yourselves together from a while back, and maybe documents showing you live at the same address, but I doubt you will be asked for them.
I think your issue might be that you're misusing a critical term. "Sponsor" is a specific term that has nothing to do with who is paying for a trip, and since neither of you currently hold a UK visa, the term isn't applicable to either of you.
A "sponsor" in immigration terms is a person (generally a family member, partner, or friend) who is already a citizen or visa holder in the country in question, and who is willing to be a point of contact and vouch for the intentions of the person who wants to immigrate.
You probably didn't fill out the form wrong, you might just be Googling using the wrong terminology.
IMHO the important point here is whether your girlfriend meets the eligibility criteria for a visa in her own right or not. If she does, there’s really little need to prove your relationship. Just state that you’re travelling together in your respective applications and cross-reference the GWF number of your application in hers. You’ll both be providing your respective bank account statements which should show you share the same address.
If you’re paying for the trip because your girlfriend doesn’t meet the eligibility criteria, you probably do need to do a little more to satisfy the ECO that you have a genuine personal relationship per v4.3 https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules. You can do this very simply by stating in the application (or a covering letter) how long you’ve been in a relationship and why you’re paying the costs of the trip eg because you are currently the wage earner in the household. You will still both have to provide bank statements, which should prove you share the same address.
for a UK Standard Visitor visa, anyone can provide funds, maintenance and accommodation. It is not necessary for the third party doing this to be a UK citizen, or to be in the UK https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/673351/Visit-guidance-v7.0EXT.PDF#page20
In UK Immigration Rules parlance, ‘sponsor’ means the person in relation to whom an applicant is seeking leave to enter or remain as their spouse, fiance, civil partner, proposed civil partner, unmarried partner, same-sex partner or dependent relative, as the case may be, under paragraphs 277 to 295O or 317 to 319 or the person in relation to whom an applicant is seeking entry clearance or leave as their partner or dependent relative under Appendix FM. https://www.gov.uk/guidance/immigration-rules/immigration-rules-introduction.