UK dividends and the higher rate (40%) taxpayer
If you do not pay tax or pay at the basic rate (20%) you will not have to pay any further tax on dividends from UK companies. If, however, you pay tax at 40% or, worse still, 50% there is additional tax to pay of 25% of the dividend received.

Needless to say the actual calculation isn't straightforward but an example may help explain it.

A. Shareholder receives a cheque for £9000, which has been paid after a 10% (£1000) tax deduction made by the company. The tax liability would be:

Tax @ 32.5% on the "gross" dividend (£10,000)        £3,250

Less tax "deducted at source"                                 £1,000 

Additional tax due                                                  £2,250