I have seen some discussions where people have shown their English proficiency for writing skills through their report writing at work - I have few reports too but they are company confidential - does anyone think that will be a problem when the officer views it? I don't work there anymore since last month but I am still contracted through my consulting company.
I don't know what other means I can show my English writing skills through my work experience.

As far as my knowledge goes, since you sign a confidentiality clause, you cannot show the reports belonging to old client to others. However, you can take the same old report, modify a little bit and remove any references to the old client data like name address, names of the locations, offices and show as examples of your work. Please make sure you modify some items so that it cannot be traced back to the old client.

i understand..i have done the same but at the same time the client I worked (for 6 years) at is a globally known company - shouldn't I mention the name of the client at all anywhere, say in a letter claiming my work experience there and hence English proficiency?
Hence, even if I remove the names of the programs, or any figures or graphs or names of persons I worked with, the officer would still know I worked at the client location.