You have nothing to lose but everything to gain. 6 - 12 months will fly by! Beijing is a great place to live, a very interesting place indeed, and they speak proper mandarin, so if you learn anything before you move out there, at least they will understand you and you will be able to understand them.
Make sure your housing allowance budget is good - $2000+ per month, 1 or 2 flights back the UK per year, having the use of a driver maybe? you will have use of an ayi [cleaner/nanny] this is standard with living in China, health insurance - I cannot stress enough how important it is to have a good health insurance policy here, make sure it covers ALL medical expenses and repatriation.
The down sides are pollution, particulary in the winter when they start burning coal, and you get the sandstorms from Mongolia, its hot in the summer [not a nice heat that you can sunbathe in, its very very humid, and the air just hangs] but most expats tend to go bac to the UK then anyway. The plus sides are, the culture - you never know what you are going to see when you step outside your front door, the parks - oh the parks, fantastic for people watching, the standard of life you can live as an expat is great, it is easy to live in an expat bubble though, if you are willing to stay out the safety expat zone you can gain so much more in the experience.
There will be support groups like Brits Abroad there, everyone in the groups was a newcomer, and groups like this are great for helping you get accustomed to the place and getting over culture shock etc.
Do it!!!