The income tax in UK has this quirk: once you fall near the top tax bracket, you retrospectively lose all tax-free allowances you could have availed, had you not fallen in the top tax bracket.

Likewise, once corporations blow past a revenue cap, they shouldn't be eligible for lower rate of corporate tax. If the US is any cautionary tale (it always is), it's a moral imperative not to let corporations get too big. Letting corporations grow big and still avail tax cuts, is like feeding your own cancer.