If the supermarkets seek to agree a common pricing policy on any item they sell (including carrier bags) they will be in breach of anti-cartel legislation and could risk multi-million pound fines.
Even if they were permitted to formulate a common policy, there would always be some areas of the country where small local supermarket groups, or individual traders, did not sign up to the policy. The 'big boys' couldn't be expected to put themselves at a trading disadvantage when compared to their competitors. (If they did so, their shareholders could force the company to withdraw from the common policy of charging for carrier bags and the whole scheme would collapse)
The only way a charging policy on carrier bags could be made to work, would be if the UK government was to follow the example of the Republic of Ireland which has placed a tax on all carrier bags. The supermarkets are prohibited from simply absorbing this tax into their prices; they're obliged, by law, to collect the tax from their customers.
Chris