If Germany, France and the UK were to announce that they’re not lifting sanctions I doubt they would describe themselves as breaching the 2015 agreement. Perhaps it’s a technical to say breach but since it isn’t a direct quote from either government it comes off as the Guardian editorialising and painting the Iran government as a victim. Their article even lists what Iran has done that would lead the UK to not abide by the agreement.

The agreement was pretty much dead when the US pulled out and then murdered one of their Generals.

The actual relevant paragraph:

Donald Trump took the US out of the nuclear deal in 2018, but Germany, France and the UK remained inside the deal, even though Iran responded to the US walkout by breaching the agreed limits on the quality and quantity of enriched uranium. Iran is closer to producing weapons grade uranium than ever before.