It was the first visit by an Iranian delegation to Britain since the two countries re-opened their embassies in London and Tehran due to a thaw in relations.
While Iran has "no fighting force, as such" in Syria, Hossein Amir-Abdollahian said Tehran had increased the number of its military advisors there to help Assad prevail against a range of opposing forces in a civil war that has that has killed 240,000 and displaced millions.
"In any political process the role played by Bashar al-Assad will be important," Hossein Amir-Abdollahian told the Guardian newspaper.
"We are not working for Assad to stay in power forever as president. But we are very cognisant of his role in the fight against terrorism and the national unity of that country.
"The people of Syria will make the final decision – and whatever decision they take, we will endorse."
Iran is a key ally of Assad that has stood by him since an uprising against his regime broke out in 2011, but has played down the extent of its on-the-ground support.
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Regional rival Saudi Arabia, which supports rebel groups trying to oust Assad in Syria, this week accused Iran of "meddling" in the region, including in Yemen where Tehran is accused of backing Huthi rebels that overran parts of the country.
Amir-Abdollahian hit back at the criticism from Saudi Arabia's foreign minister Adel al-Jubeir.
"Considering the fact that for seven months Saudi Arabia has been attempting to occupy Yemen using force, when it comes to Syria they are not in a position to make such comments," Amir-Abdollahian said.
"I recommend that (Jubeir) instead of passing the buck considers cooperation and constructive behaviour in the region."
Amir-Abdollahian held talks with British foreign minister Philip Hammond and his deputy Tobias Ellwood on Wednesday.
Ellwood wrote on Twitter that the three had "discussed our positions" on Syria and Islamic State militants who have brutally carved out regions of control in Syria and Iraq.
The foreign office declined to comment further on the meeting.
Britain re-opened its Tehran embassy in August, nearly four years since it was closed after it was stormed by protesters in an anti-sanctions protest.