Calculating how much a company or a country pollutes is not really straightforward. For example pollution caused when a product is manufactured in another country is not included in the UK's emissions. Nor is the pollution created in transporting goods to this country. So the more we use goods that are made in China (for example) the lower our emissions will be. And of course the higher China's will be - even though they are exporting what they make, and everything is used in this country.
Have a look at these figures published on Our World in Data showing the difference between consumption based figures and production based figures. The UK would have emissions 42% higher if all the emissions embedded in trade were included. That's up from 10% in 1990. So we are pushing more of our CO2 use onto the carbon budgets onto other countries.
One of the first steps countries could take in trying to keep average global temperatures to under 1.5° is to tell the truth about where the CO2 that is going into the atmosphere is actually being used. It seems hard to imagine a binding agreement across all the major economies (and the major emitters of CO2) without that.