there are two problems here.

1. Tax: If you need to transfer the money to any UK bank ( it seems to be exactly the case as you need the sales proceeds to pay for your flat here) then you need to declare it and pay tax. Because before any bank can accept the transfer, they have to check the source for anti-money laundry purpose and they would ask you to show evidence that the money is indeed the proceeds of selling your flat in China. Once the money is in your personal account, there is no way that you can avoid declaring it. 2. But, a big BUT, how can you transfer the money out of China using legal means?

One of the tax-efficient solutions could be creating a company in China to protect the money and using that company to buy any UK property.