Because a flat rate tax will put Britian on the map again. Ex Pats will pour back home bringing with them considerable capital since most are high income earners. Even if they don't spend all their time in the UK, they will be happy to be tax domiciled in the UK again. People that have been disconnected from their families and friends will suddenly see that's its worth to return to their homeland that treats them with respect not as a cash cow.
Businesses will return to the UK, plenty have been moving down to Switzerland lately to avoid the communist regieme installed by Brown. But who wants to live in Zurich compared with London ?
In 1997 when Blair came in, Britain entered a kind of renassiance period for some years, when suddenly London was cool again, but like much of Labour it was a fake. Just like the 60s and 70s labour govts. when the joy ride of spending came to an end the hang over begun.
Make a flat tax, and you will return the UK to being the global business centre that it always was. Make the annnouncement and see if the USD / GBP doesn't jump 10 cents in one day !.
You'll be way ahead of the EU countries, way ahead of the US, all the innovation and entrepreneurs will pour into the UK bringing jobs, and cash with them. Do it now and do it fast, before someone else does !!!.
And before you write this idea off, look at the burden of taxes, the budget for collecting them and running the Inland Revenue which is all ulimtately useless money. Look at the expenditure of businesses on accountants and book keepers, and storing huge piles of documents for ever and a day, all wasted useless money, going into a huge complex bureaucracy that no one understand properly.