Difference between 'PhD by Published Work' VS 'h.c. PhD' VS 'Dr hab'? I have just learned of the 'PhD by published works' which appears to be a fairly common program for schools in the UK. Is this similar to an honoris causa conferral or is it some separate program? 

After reviewing materials on the Oxford Brooks &amp; Warwick sites it appears that the entire program is just preparing a defense, which sounds a bit like Habilitation at first pass but it also seems that these programs are not restricted to current PhD holders. 

Can anyone shed a little light on this for me?

Thanks.
While it may be common for universities in the UK to offer such a degree, I'm not sure it's at all common to obtain a PhD in this way (i.e. 'by published works').  I think it is mainly aimed at people who have been involved in research for some time (especially as university staff members) without a PhD, but who have published material equivalent to a PhD thesis.  (As you will have read, for the Warwick program one must be either a member of staff at Warwick or have graduated with a Bachelor's degree or equivalent at least seven years ago.)
Generally someone applying for such a program would have already published the works they are planning to submit, and therefore as you say the program will consist primarily of preparing for the oral examination (often called 'viva' in the UK).  Although you probably have to prepare some kind of extra document as well (in the Oxford Brookes program this is a 'critical appraisal' of the works being presented).

This is, as Pieter has already explained, very different from either a habilitation (which is at a higher level than PhD) or an honorary PhD (which in general can honour any kind of accomplishment and doesn't require the holder to have done research at PhD level or indeed to have a university education at all).