In an earnings presentation in late July, EDF said the increased risk of a 15-month delay is due to "performances on civil works and challenges on mechanical, electrical, heating, ventilation and air conditioning" and "progress is below the planned trajectory and action plans have been set".

Translation: It's taking longer than we thought.

EDF has targeted June 2027 as the first operation of Unit 1,

47-months out, and they're talking a 15-month delay, taking it to Nov-28 for Unit 1, never mind full operation. Increasing the remaining time by ⅓, and I'm sure it will come with increased costs.

How much green infrastructure could we have built with this money?

Wikipedia:

Since construction began in March 2017, the project has been subject to several delays, including some caused by the COVID-19 pandemic,[10] and this has resulted in significant budget overruns. As of May 2022, the project is two years late and the expected cost is £25–26 billion,[11] 50% more than the original budget from 2016. It has a projected lifetime of 60 years. In February 2023, EDF announced that costs would rise to £32.7bn and operation would be delayed by a further 15 months to September 2028.[2][12]

So this is potentially the second 15-month delay this year? At that rate this plant will never complete.