http://www.pm.gov.uk/output/Page10873.asp contains the response:
We received a petition asking:
"We the undersigned petition the Prime Minister to tax disposable plastic carrier bags to recreate the effect of the Irish 'plastax', reducing dependence on foreign fossil fuel supplies, reducing carbon emissions and reducing the volume of landfill waste and litter in our towns and cities."
"Although plastic bags only form a very small proportion of household waste and litter, as you know they can be particularly visible when discarded in the local environment. The Government is therefore committed to exploring ways to reduce the impact of disposable plastic bags on the environment, and welcomes efforts to support the sustainable development of biodegradable plastics from non-fossil sources.
A plastic carrier bag tax would significantly reduce the number of single-use plastic bags in circulation. There is, however, no clear evidence that such a tax would be beneficial on either broad environmental or litter grounds. This is because the overall environmental effect would depend largely on the nature and extent of substitution by other products, including the use of bags made from other materials, as well as alternative forms of packaging. For these reasons, the Government does not have any plans to introduce such a tax at this time.
The Government is, however, exploring other ways to reduce the impact of plastic bags on the environment. For example, through its Waste Implementation Programme, the Government has piloted a Reusable Bag Campaign with retailers, aimed at reducing the consumption of plastic carrier bags by promoting the message of reuse to the public. Working with the Devolved Administrations, the Government has now approached the major retail groups to discuss joint solutions for reducing the overall environmental impact of both paper and plastic bags.
The reason for the joint focus on paper as well as plastic bags is that if initiatives are only introduced in relation to plastic bags, this may result in a switch to paper instead. Whilst paper bags are derived from a renewable resource, the manufacturing process for them is resource intensive. Considerable quantities of oil are used, particularly for the extraction of the raw material, wood pulp and for transportation. As paper bags are heavier than plastic bags, they therefore require more energy to transport and result in greater emissions."
And they provided a link to: http://www.defra.gov.uk/ENVIRONMENT/WASTE/topics/plastics.htm