I've mostly worked in public-sector bureaucracies, not software companies, so my experiences won't line up exactly. In each case, the rule has been strictly pro-rata. I've generally felt that the company has had the good end of that deal, especially for the higher fractions (0.6/0.8 of full time): productivity seems to me to be non-linear, and trading off the end-of-the-week burnout for a little more payroll admin an…
Not familiar with UK employment practices, but: do the businesses pay any benefits to part-time employees?
We don't offer private health care currently, and every other benefit is a pretty direct link to how productive that employee is - technical books, better work environment etc.
For instance, we do a "wonder week" once a quarter to work on side projects/learning etc. To me, not letting a part-time employee do this would be to treat it as a kind of bonus, when instead we view it as an opportunity to explore ideas and skill up.