ActiveX controls work in IE8, so they probably wouldn't need to be rewritten or ported. But we'll never know. The UK government is short circuiting the process by asserting just the act of testing would be too burdensome. I call bullshit.
[disclosure: I wrote that blog post, so consider my opinions here highly biased. :-) ]
I've spoken with another developer or three that tell me that, although ActiveX controls remain supported, security policies have changed in some meaningful ways between versions of IE. The result is that your controls that were working fine in IE 6 stop working in later versions of IE. I have not personally confirmed this (and god send that I will never have to).
[disclosure: I wrote that blog post, so consider my opinions here highly biased. :-) ]