Britain, along with the U.S.A., is war weary, and after the travesty of Iraq and Afghanistan, has grave misgivings in any future involvement in the Middle East. The ghost of Tony Blair and his single-minded determination to attack Iraq, at any cost, has cast a long shadow over British politics. The British public have a long collective memory.
The Syrians lost out because they happened to have their revolution during an election cycle in America, France, and bizarrely, some kind of election cycle in Russia. Obama wasn't going to be seen to commit anything to Syria, that would be political suicide, but he's also got a Nobel Peace Prize sitting on his mantelpiece.
Britain is one of the biggest arms dealers in the world, after the States. There's a lot of money in weapons deals. We sell weapons, and our responsibility ends as soon as they leave the country; they're unpacked, and then before you know it, they're spread around the world.
The Saudi government uses a lot of British equipment to suppress their own people. But we're happy for our politicians to go on advertising trips to Saudi, selling our weapons at the trade conventions.