45% is for extremely high earners. 40% is any income above around the £45,000 mark, maybe a bit higher. CBA to check exact figure. Income from around £11,000 to £45,000 is 20%.
I think the "economically correct" way to calculate the tax burden on employment is to calculate the government's take as a percentage of what the employer pays out. In other words, you need to include NI, both employee and employer. I last calculated this a few years ago, so my figures may no longer be bang on, but I believe the rate is in the region of 38% for a basic rate taxpayer and 47% for a higher rate one. I've never calculated it for top rate but it's fairly clearly going to be over 50%.
I do believe the rates calculated above should be capped at 50%. SInce they are in fact so close together, you could simplify things and tax everyone at 50% while increasing tax-free allowances at the bottom, so that overall you still brought in the same amount of money. You could also tax pensioners and people living on investment income at the same rate as workers, that would actually bring in some extra money, as they don't pay NI now.
Removing the smoked and mirrors of NI would let people see how much they are really taxed, and if that happened they wouldn't stand for current levels of tax, let alone higher ones.