This could repeat a few times, but in the end it would only have cost a lot of money and slowed down main street adoption of crypto currencies for a while...
It would be pretty crazy to have the government come out in public and declare the war on bitcoin or crypto currencies, with the possible encryption ban in the U.k could that be the begining, from the war on drugs now to the war on encryption and bitcoin.
It would definitely be crazy if it happened, and an encryption ban might give them cover to do it. I also think if a government saw significant tax revenue being lost to anonymous cryptocurrency transactions, it is not beyond the realm of possibility that they might attack the network directly.
I don't know how significant can be cryptocurrency transactions within a single country when on a global scale the bitcoin has a 4Billion USD market. Is the encryption ban directed towards cryptocurrencies? It sounds to me more like the government wants to listen to peoples internet calls, read emails and more. If banks wanted to bring down bitcoin, I don't think they would need such measures. Why not just ban cryptocurrencies then? Cheaper than attacking it.