HI Dear,
I am not sure why you initially informed them that your employer can not provide the information, that's something which raises question against your experience credibility, anyways what has happened has happened, the answer you mentioned above is bit in technical in terms of documentation as i can't understand - like UAN card EPFO portal. I know you are sure that the best thing to provide them as a proof.
So, i believe any work permit, offer letter, labor contract (usually this is GCC concept to issue labor contract), even the photos, appreciation letter from the company, many company provides certificates of 5 - 10 years service (if you have them), there are so many proof you can provide to them - but you are the only best person to answer this question that what you have right now in hand, you might need to go back to your old email's, archived files, any communication between you and your HR, that all works as proof.
and the best thing is that if you are in the good contact with your HR, let them write an email to you and confirming your status, my question haven't they provided you the work experience certificate?
You need to think like the visa officer, just think from his side, if you gonna provide him any of those documents, what will be his reaction, if he is going to accept or not, if not why, you might find an answer by yourself.
You need to provide all the possible links with your employment historically, think about that.
Let me know if you require any other advice.
Hope this helps.