In message , Neil Williams
writes

It is not a tax; it is a road toll.

"Toll: A tax or duty paid for the use of a public road ..." (OED)

Do the embassies also not pay to,
for instance, cross the Dartford bridge?

The crossing is free for vehicles exempt from vehicle excise duty, so it
is quite possible that diplomats don't have to pay. However, it is a
rather different issue - there's no compulsion to use the Dartford
crossing, whereas most embassies can't avoid the Congestion Zone in the
course of their work, even if they think they can avoid paying for it.
--
Paul Terry