LONDON — The Russian flag has been there so long now that it hardly attracts any notice, just another familiar piece of background scenery in the global, cosmopolitan Premier League.

It hangs from the upper deck of the Matthew Harding Stand at Stamford Bridge, the home of Chelsea Football Club. In its central blue band, spelled out in white block capitals, are the words “The Roman Empire.”

It is not intended, of course, as a political symbol. It is not demarcating this patch of West London as sovereign Russian territory. It is simply a display of gratitude to Roman Abramovich, the Russian oligarch who arrived at Chelsea in 2003 and, cracking open his sizable bank account, quickly turned the club into one of soccer’s modern superpowers, a serial champion of England and, at its 2012 peak, the champion of Europe.

And yet, as relations between Britain and Russia strain and crack in the aftermath of the poisoning of a former Russian spy and his daughter on March 4, that flag becomes a symbol of something else — how difficult it would be to punish Russia’s ruling class by separating its oligarchs from their property in Britain.