De-Facto Relationship switching from WHV to HSMP dependant

Is it possible for an unmarried partner in the UK on a WHM to switch visas based on a de facto relationship with a HSMP holder even though the relationship was not mentioned on HSMP application or FLR app?

When did this relationship begin?

We have been together about 4 years. But only living together once she moved to the UK, which was 6 months after I arrived.

We will only have lived together for 2 years as her working holiday visa expires, which is roughly 6 months into my HSMP visa.

Thanks for your help.

You must proove to the satisfaction of the Home Office that you have been in a relationship 'akin to marriage' for at least 2 years prior to the date of the 'FLR on a dependent basis' application as evidenced by the submission of documentation evidencing said co-habitation at the same address including but not limited to:

1. Financial - bank accounts, credit cards, wills, insurance policies

2. Utility bills - gas, electricity, council tax, water, service charges

3. Government documents - Inland Revenue (P45, P60, PAYE TAX CODING), NHS Cards

4. Driving licences

5. Employment docs - payslips, employment contracts