So my 5 minute google search, heavily influenced by a blood alcohol level beyond all reason produced this: http://www.state.gov/strategic...
My reading indicates that if anyone is selling the particular xeon's in question to you (note that I believe not all Xeon's are controlled, just a subset), that either your country has treaties with the US that suggest they will in fact come down on you for selling to China, or the seller is committing a US crime and will be penalized for selling to you. I could not sell these to you now that I know your intent, for example. Since the US and UK tend to be as close as the US is to anyone not US, I'm going to assume the UK has the appropriate treaties and will in fact come down on you for reselling this to China. While I totally understand if that upsets you, and I feel much the same when the US government obeys UN regulations *I* don't approve of, it's how The Man works.
As I said, I don't really think you'd necessarily be caught, and I'm confident China will be able to get proc's for it's supercomputers regardless from someone. The issue really is about economic sanctions making it more difficult for China to be a producer of Intel-based server systems, which actually does hit them in the wallet, given how much Foxconn, Quanta and MSI do in the mfg space. China has a big business in the low level PCB mfg & board assy business, which it wants to expand into design & systems, but in this case may not be able to do legally if it continues to pursue nuclear tech. In a similar note, I doubt say, Iran, has any difficulty getting the latest Intel server for it's government operations. But the majority of the country is deprived, and the market is defunct.
Again, I don't care, I am entirely disgusted with anyone having a relationship with China that doesn't involve arm twisting and threats of some form of annihilation, but I feel your anger here is misplaced. I'm with you when some idiot senator decides she wants to erase the anarchists cookbook from the internet and will mock her to infinity when she tries to enforce her idiocy outside the US borders, but in this case we're dealing with China, and fuck China. Call me back when they have some form of believable democracy and even a hint of something like the magna carta. That was from the UK right? I thought your ancestors (and mine, as it happens) thought that was a big enough deal to die over, I'd hate to see it tossed out for cheap shit and rich people getting richer, or an only slightly misplaced anger over my country's ability to bully, which in this case may actually be to our collective benefit. The fewer people with effective nukes, the less likely the world ends tomorrow.