Do you have a joint bank account? Do you live together, and have you for 2 years? Do you have any shared bills?
When I got married, we looked into all these things and we just didn’t have enough proof to meet the criteria (we’d been together for 5 years as students, but only spent 1 year living together with joint bank accounts and that just wasn’t enough to meet the burden of proof). It’s only got harder since then.
We submitted a big dossier just for our marriage visas – joint bank statements, proofs of address, all sorts.
Feel free to get in touch with me about it if you like – we did ours right at the start of Theresa May’s and it was absolutely awful. Good luck.