I assume that when people talk about 'predistribution' they mean the distribution of what ONS refer to as 'original income', which for people of working age is basically income from earnings and investment, before taxes, National Insurance contributions, government income transfers and benefits in kind. Targeting 'predistribution' as advocated by Labour leader Ed Miliband would therefore, presumably, involve trying to shift the distribution of original income.
So how has the distribution of original income developed over recent decades?
There are various ways of summarising the changes: the chart at the head of this article is just one of them. It shows the shares of original income received by the bottom 50% of /non-retired/ households compared to the share of the top 10%. What it shows is that since about 1990, the latter has exceeded the former. In 1977, when the data starts, the bottom 50% had 32% of original income and the top 10% had 19%. By 2010/11, the original income of the top 10% was 30% of the total, while that of the bottom 50% was 21%. (The source is ONS's historical data on 'The effects of taxes and benefits on household income http://www.ons.gov.uk/ons/rel/household-income/the-effects-of-taxes-and-...)
I think the data in the chart comes as close to speaking for itself as any summary statistic ever does. The UK 'predistribution' is very unequal now compared to the 1970's, but most of this shift occurred in the 1980's; from the mid-1990's to the onset of the financial markets crisis, relative shares didn't shift that much; but over the last two years, there has been a trend towards further polarisation.
My only comment for the time being is that this bears out one part of Ed Miliband's critique of the previous government's approach- the distribution of original income really didn't change much under Labour (although there were in fact some modest increases in the income share at the bottom of the distribution)- but the general shape of the distribution is very longstanding. Targeting 'predistribution' is not going to be an easy option.