First, I would like to appreciate your efforts. I am glad that at least you are fighting.
Software Companies and Fake Letters
Let's come to your point. First, let us see what software companies can do and what they can't do with employees with fake letters. The National Association of Software and Services Companies (NASSCOM) is the largest organization of the Government of India to govern all these things. They have a database called the defaulter database. Any company can provide necessary documents for updating defaulter names in the database. It requires loads of evidence. If proven, a letter will be issued to the defaulter regarding their blacklisting, and their name is updated in the database.
For example, Infosys kicks off 10-15 percent of employees every year due to fake letters, but they have hardly taken any action yet. The maximum they have done is that the person would not be able to apply to Infosys again.
Your Case
Now, come to your case. If your uncle has a registered firm in London or anywhere and is providing you with experience, then no one in the world can prove it wrong. Mark my words—if he has a registered firm and is willing to verify your certificates, then you don't need to worry at all.
You have cleared the interview; you deserve it. Don't worry at all, chill