Ah yes, Laffer's elusive curve!
It's a truism that at 0% and 100% tax rates there will be no revenue, so there must be some form of relationship between these two values. Where the turning point is seems to depend on which end of the political spectrum the researcher lies with right-wing think-tanks finding that tax revenues start to fall at lower tax rates than social democrats. These two articles from Time and the Wall Street Journal exemplify the range of opinions:
http://www.time.com/time/magazine/article/0,9171,1692027,00.htmlhttp://blogs.wsj.com/iainmartin/2010/08/29/danny-alexander-meet-the-laffer-curve/The turning point (above which higher tax rates only result in lower total revenues) is going to depend on a number of factors, including the effectiveness of the tax administration, the number of loopholes in the tax code and, of course, the mobility of the tax base. Which side of the Laffer curve is the UK on? In my opinion (obviously not supported by the Adam Smith Institute!), the 50% tax rate is still set low enough to yield higher revenues and satisfies Colbert's dictum of yeilding the maximum quantity of feathers from the goose with the minimum amount of hissing. This seems to be the Government's view as well. This opinion is borne out by the evidence on the impact of the Reagan and Bush tax cuts, which merely weakened the fiscal position (probably their ultimate, if undeclared goal - see the Time article mentioned above, including the quotes from Laffer himself)
The mobile super-rich will already have made their arrangements whether the top-rate is 40% or 50%, and with the UK's ridiculous 'non-dom' rules the rate of tax is almost irrelevant for these people anyway. Incidentally, I heard the other day that super-rich immigrants to Switzerland can negotiate the rate of tax they are willing to pay with whichever canton they might to choose to reside in (final choice being heavily influenced by the outcome of the negotiations!). I don't think the UK should be joining this 'race to the bottom'.