You do not have to upgrade to TV M+. You can stay on TV M.
Paying the £49.95 activation fee and the £20 self-install fee will allow you to watch the HD channels which come with TV M. See here:
http://shop.virginmedia.com/existing...-channels.html
The £20 charge covers the cost of postage and package for the new box and return postage and package for your old box.