It will be based on output emissions and what standard the car was certified to
If certified to EU4 then it could be a good challenge to TfL
If not then regardless of capability they will treat it as EU2
Stuck in your reg into the government vehicle tax check website will tell you what emissions level it was certified to
As an example the engine rarely changes across a range but the exhaust after treatment will which means the same core engine can be certified through from EU2 to EU5 back in the day