What is the quality of this internal speaker wire?

It is labelled "Van Den Hul H.C. CS 18 Nitrogen Free".

I have it as internal wiring in my Tannoy D-700's and was wondering what level of quality it is.

Would upgrading it change/improve the sound/characteristics of my speakers?

I would leave it. Tannoy does a fine job of voicing around the VdH cabling they use. The VdH cable is some of the best valued cable out there and we should all be so lucky if all speakers had that grade of cable for internal wiring.
Yes, upon further research, I find it is oxygen free copper stranded wire that is silver coated. It would be hard to get a sonic improvement worth the time and money it would cost.