There are several ways to compute the SVD in OpenCV:
cv::SVD homographySVD(homography, cv::SVD::FULL_UV); // constructor
// or:
homographySVD(newHomography, cv::SVD::FULL_UV); // operator ()
homographySVD.w.at<double>(0, 0); // access the first singular value
// alternatives:
cv::SVD::compute(homography, w); // compute just the singular values
cv::eigen(homography, w);
Have a look at the documentation for cv::SVD and cv::eigen for more details.