Missing the point
It's actually the bank that will pay the tax, not the employee. The employee pays normal high rate tax, any bonuses above £25k are added together and the bank pays 50% of this total separately to the amount paid to staff, the upshot is that the individual pays their normal amount of tax.
This is clearly a populist vote winning move on behalf of the government who are trying to make it look like they are hurting "the bankers", it won't hurt the individual but will cost the banks. The banks aren't able to just not pay bonuses because, if someone has reached their required targets for the year and isn't paid their bonus, the bank aren't paying fair or competitive pay. This then means that the banks are in breach of the companies act, which requires the board of any company to always act in the intersts of the shareholder, by not paying competitive pay they are not acting in the interest of the shareholder and will have to resign or face legal action. This is what nearly happened at RBS recently when the government suggested that they would ban RBS and Lloyds banking group from paying any bonuses. Instead the government quietly dropped their 'ban bonuses' policy and came up with this new tax on banks not bankers. If they wanted to ban bonuses, they should have nationalised the banks, so there wouldn't be any shareholders to not act in the interest of.
People should try to realise that if you work in a bank, you're not always a banker or trader. Many people are required, IT, Security, Cleaners, Facillities management, HR, etc. etc. and they are all being tared with the same brush.
PS Don't have a go at me for the above, I'm just saying how it is, not expressing an opinion. Almost no news organisation has bothered to properly explain how the above works, except Radio 4, gawd bless 'em.