33% is not correct.
There is 20% Income Tax, 12% Employee's NI, but also 13.8% Employer's NI. Employee's NI goes down to 2% above £40,040 per year, but employer's NI is not capped.
So if you have someone on £20k/year, and you give them a £1k pay rise, they will receive £680, but you as the employer will pay 13.8% NI on that, so the cost of the payrise is actually £1,138
Therefore the actual tax rate is not 32% , but 40.25%
For higher rate tax payers, you receive £580 out of £1138, so the actual tax rate is 49%.
If you earn over £50,000 and have children, you get your Child Benefit removed, at a rate dependent on the number of children you have, but for two children currently 17.52%, so an overall 66.2% tax rate.
On incomes just above £100k, you lose your Personal Allowance at a rate of £1 in £2 earned. So the effective tax rate is 66.6%.
For those on the 45% band (above £150k), the effective tax rate is 53.4%.
Obviously tax rates on earned income are far too high, which is why only the little people bother with them, and the likes of Jimmy Carr and numerous others set out to avoid paying tax entirely.