Art. 29 of the Vienna Convention on Diplomatic Relations provides for the personal involability of a dimplomatic agent: no arrest, no detention /1 
If an ambassador drives under the influence (as an obvious example of what’s not ok) the police will stop him, prevent him from driving on to protect the public, but not detain him.
What Iran did seems to be an obvious violation of Art. 29. /3 theguardian.com/world/2020/jan…
A country is not without remedies against a diplomatic agent - when worst comes to worst the agent can be declared persona non grata. But not engage in other countermeasures. See the Teheran Hostages case before the ICJ /4
UK statement
Latvian statement
Statement by the UK ambassador
