Why can't you bring an iMac with you on the plane? I've brought over three 22" LCD screens in the past - they just take up an entire double box with all the stiff styrofoam you need for cushioning.
The first time I flew one over, I tried to declare the monitor and pay duties on it. The airport customs official seemed visible upset that I caused her to have to do some work and fill out forms. So the subsequent ones went undeclared.
As for the rest of the shipping manifest, if you ocean freight a whole container, it's a crapshoot whether they even bother opening up the container. I declared a few items and paid a few bucks just to seem like a typical household move and they never even opened up the container (the original shipping seal was still intact) to find all the other goodies that could be dutiable.
Also, when the US packing team writes up the bill of lading, get them to use as vague description as possible. My packers inventoried my disassembled $4000 digital piano as several boxes of non-dutiable keyboards - after all, the original box printing said digital keyboard - not digital piano

In your case, get them to inventory your iMac as just "desktop computer" - the typical desktop computer is only going to be worth USD $500 resulting in under £100 duty if they don't open things up and look too closely. Of course, take pictures while they are packing so you can prove you really did have an iMac if you ever need to make a shipping insurance claim...
I thought the computer goods duty was closer to 20%, but that was five years ago. Anyhow, I highly doubt you would have to go to Beijing just to get a Mac. There are Chinese retailers that can do the legwork for you to buy a gray market iMac in Hong Kong and lug it to bak to you in China without duties. They also deal with shipping back and forth if you ever need warranty service.