You would probably be best taking BA to PEK (maybe PVG) then flying onto Xian from there - All sorts of Chinese airlines cover the PEK to SIA route - china southern, china eastern, air china. But none are OW AFAIK. internal flights are fairly cheap and reliable. You would not have enough miles to fly CX to China.

With 50k miles you will only be able to get an economy return in BA. Don't shoot the messenger but you will find this is not all that cheap, and probably not a good use of your miles. You would need 75k for WT+ which would be better value. You will be still better off going for a paid WT+ ticket and using 25000 of your miles to upgrade to club, but it maybe that this is not what you are looking for. You could do a 'miles and money' redemption, but I would not recommend it

Eg WT+ apx £660 + 52500 Avios
WT+ £470 + 75000 Avios
WT £360 + 50000 avios