Marginal Net Outcome
Disclaimer:
Nothing has been checked really. Could be quite wrong in the details.
TL;DR
If you can choose how much you work and receive gross income at a linear rate, your marginal return per working day decreases substantially as you work more due to progressive taxation and pension contribution limits. The UK tax system creates significant "efficiency cliffs" where working additional days becomes much less rewarding.
Marginal Net Outcome
Channeling Dan Niedle at Tax Policy Associates, let's understand the tax efficiency of working different intensities.
Net Income + SIPP Contribution = Net Outcome (NO)
(treating pension as income since it's money you keep)
Change in NO per Additional Working Day = Marginal Net Outcome (MNO)
(how much extra you get for each extra day worked)
Calculate your marginal net outcome rate per working day at different work intensities. This shows how much additional net outcome you get per extra day worked due to progressive taxation and pension limits.