Here's the up to date info on biodegradable bags and charging :
Super biodegradable bags
There are no exemptions for biodegradable bags at present. However, the government is considering an exemption to encourage development of a new, genuinely biodegradable, more environmentally friendly bag. This would be introduced as an amendment at a later date.
Defra has commissioned a review of existing industry standards for biodegradability of lightweight plastics. It will review whether there are standards that could be used to introduce an exemption from the charge and, if so, how the exemption would be carried out. Defra has to submit the report to Parliament by 5 October 2015.