Buying iPhone 4s in China

I'd like to buy an iPhone in China for use in the UK with a British SIM card. I'd like to know if it's possible to buy an iphone for use with my own SIM card and if the guarantee will still be valid in the UK. I'd also like to know if it's possible to switch the SIM cards over, for example when I visit Spain, could I change the SIM cards over and use my local spanish SIM for the duration of my trip?

Thanks in advance