We saw a similar drop in Scotland a couple of weeks ago, and that drop has now shown through on hospitalisations. So, the case drop there was definitely real.
The fact that the case curves across the regions are similar may be significant as well. If it was just about testing rates then you wouldn't expect that. Maybe the Euros ending, schools closing, students going home and the good weather arriving have all combined. Plus, lots of people quarantined by getting tests or as contacts maybe taking a bit of the heat out - we'll also see an increasing effect from people getting vaxxed in June/July as well.
That said, it does look like the Euros behaved like a bunch of super-spreader (and mini-super-spreader events in the case of families/friends getting together for the matches). There's plenty of potential for those with the new openings, but it is impressive how fast the numbers rise/flatten/fall with Delta. Maybe something to do with symptoms arriving earlier (after 4 days?) - maybe the infectious period tails off early as well?