SINGAPORE - SHANGHAI - HONG KONG/LONDON

Hi
I'm a UK passport holder, currently in Singapore on a business trip. Originally planned to fly to HK and then back to the UK. Have been just told that I need to fly to Shanghai tomorrow and only then to HK. Could you please advise if I am allowed to
1. stay in Shaghai for two days without a visa?
2. fly out from Shanghai back to London and not HK? Would the UK, my country of residence, count as 'third country' and therefore qualify me as a transit traveller?
Many thanks in advance
Katia