3rd country, 144 hours tourist transit policy

i am a U.K citizen flying out of London Heathrow to Singapore, I have booked 4 nights hotel accommodation in Singapore before joining a cruise to Shanghai. This cruise stops in Vietnam & Hong Kong before arriving in Shanghai on the 3rd of June 2019. I have booked 3 nights hotel accommodation In Shanghai & will not be travelling outside this area. I have a flight booked, to depart from Shanghai airport to London Heathrow on the 6th June. Does Singapore class as a 3rd country & is therefore covered under the 144 tourist transit policy even though i will be returning to the same Airport in London as i originally flew from or do i need to purchase a tourist visa?