[jor]

Honest John, who is to be believed in all things relating to internal combustion,
says:
'The ULEZ is really aimed at discouraging diesels in the capital, so only diesels that are Euro 6 will be exempt.
It's more relaxed for petrols, which have to be Euro 4 or better, which is generally most cars registered after 2005 and some as early as 2001.
All hybrids, PHEVs and electric cars are exempt. As for LPG conversions, it depends on the individual model and engine, so it's important to check with TfL first. The emissions are taken from the V5C data held by the DVLA.'

jor