Obama will certainly give Cameron all his support as far as implementing such nonsensical laws *in the UK* are concerned. Why? Makes it easier for him to snoop in areas where the UK don't grant the US of A pretty much unhindered access already. The UK could render itself into a gigantic wiretap for all things concerning the EU. Everybody, except from the US of A, loses.
You really have to wonder where Cameron got this idea from that encryption is a bad thing worth banning. You'd think that a Prime Minister would have several industry experts as advisers, or spend some tax funds on getting expert advice to validate ideas before blurting them out to world & dog and making himself look like a Norbert.
Either he thinks he knows better, or he didn't understand what the advisers were saying, or he is so desperate and grasping for straws because his campaign advisers see a very gloomy future for his political career in general and the next general election in particular.
Either way, it contributes sod all to national security. $TERRORIST doesn't exactly abide by the law and is unlikely to communicate online in the UK while plotting. It's wrong (and dangerous!) to assume that $TERRORIST is stupid. Otherwise you'd have to assume that all politicians, police, three-or-four-letter agencies are even more stupid, since they haven't been able to prevent any terrorist attacks, even with extended snooping powers introduced since 9/11.
$TERRORIST wins. They managed to send politicians in the western world into a frenzy which led to a huge impact on privacy over the last decade. (Although the number of terrorism victims is likely much lower than the number of fatal car accidents, road safety has not received much attention. If you want to win votes, fix the bloody potholes in the country already!)
Any online comms outside of the UK would still be able to use encryption legally, anyway. There's absolutely no single reason why outlawing encryption would be a good idea. (But maybe the US of A have initially planted that idea within Tory ranks... because they win.)