UK Corporation Tax is 19% on profits up to GBP 50,000 and 25% on profits over GBP 250,000. In between, Marginal Relief tapers the bill, so each pound of profit in the GBP 50,000 to GBP 250,000 band is taxed at an effective 26.5%, higher than the 25% headline.
Up to GBP 50,000 (small profits rate) - 19% - 19%
GBP 50,000 to GBP 250,000 (Marginal Relief) - 25% less Marginal Relief - About 26.5% on each pound...
Over GBP 250,000 (main rate) - 25% - 25%
Marginal Relief fraction - 3/200 - Applied to (upper limit less profits)
Plus 4 more in the full guide

freedomisntfree.co.uk/guides/corporation-tax-uk-guide