No wonder George Osborne announced some crackdowns in tax avoidance.

Here’s an amazing fact. Apart from the leap in the personal tax allowance, what was the largest annual tax cut today? Expected tax avoidance this year.

In fact you may have missed the mini fiscal stimulus at the heart of this Budget. There will be a £3bn fiscal loosening over the next year, followed by a £3bn tightening in the following years. What may be surprising is that this is almost entirely caused by £2.4bn of tax avoidance from Britain’s rich this year, that is then unwound in later years. Yes, this is the OBR’s expectation in this financial year that the rich will not pay out £6.5bn of dividends and bonuses in this tax year, but shift it into April 2013 when it attracts the 45p tax rate. Perfectly legal.

This follows on from that truly amazing staistic that I revealed on Channel 4 News on Monday. The Chancellor confirmed in his speech that Britain’s rich moved a staggering £16bn of dividends and bonuses. The HMRC report says £16-£18bn. My report on Monday put this “forestalling” at £18-20bn.

So the fact remains that the decision on the 50p rate was made on the basis of one year’s highly distorted data. Now the chancellor’s take on this was that avoidance at this level shows that the tax didn’t work. But what he didn’t say was that the forestalling effect was a one-off. The HMRC report does try to strip out the impact of forestalling and analyse other “behavioural impacts”.

The OBR’s best estimate is that the behavioural impacts, such as retiring early and emigrating, etc, are far bigger than the Labour Treasury had assumed. That is key to understanding why the cut from 50p to 45p is scored at such a small giveaway to the rich.

There is a mountain of uncertainty about that. OBR boss Robert Chote himself told me the following when I pressed him about it:

“If the government decides it’s going to cut this tax rate, then we have to make a choice – we have to make an assumption about how people are going to respond to this .. and the implications for our own forecasts.

This is a judgement based on not even a full year’s data based in terms of how people have responded to the 50p rate, in particular in terms of those self assessment tax-payers.

The costing of these sorts of changes is by no means unarguable… you have to disentangle colossal forestalling. Then you have to work out the long standing impacts.”

None of this uncertainty suggests it is right or wrong to cut the 50p. It’s just almost impossible to discern from the existing data. It might have been that this year’s data, a full, far less distorted year of 50p might have yielded considerably more tax. It was in a way strangled at birth.

Actually it really was nobbled from the start by the fact that it was pre-announced by Labour with a year to forestall income. In some ways it was set up to fail. Many economists will argue that it was always going to. We’ll never know.