Responsible for what, exactly? They could certainly be held accountable under the trades descriptions act for false advertising, I would imagine, if any of their products, described as 100% beef were found to contain horsemeat - which is why they have, as a precautionary measure, removed all such products from their shelves.
A wise, and reputationally required move, I would have though- although a move that will cost them, at least in the short term.
What all of this story suggests is that that the consumer is hostage to an extended global food chain, some elements of which may elect to resort to frankly illegal actions in order to maximise their profits, and we simply do not have sufficient checks and balances to properly protect the consumer.
It should be mandatory for DNA panel testing to be carried out on all meat destined for human consumption to establish the provenance.