I'm trying to set this up at my current job and my expectation is that those extras hours get to be 'focus time'. As I become a more senior developer and have to take on more managerial responsibilities my time to actually get my own work down is disappearing.

I started working ~1.5hrs early in the morning just so I could get my coffee/hackernews routine out of the way and really get my head in to the work, then by the time stand-ups and meetings start I'm ready for a break and helping my team. Do an extra 30min at the end of the day wrapping things up and there's your extra two hours being used productively.

I'm actually already working this schedule (5 days/week) due to lack of senior staff at the company, I just want to formalise it and get Mondays off...

If I was purely coding my own tasks all day I would probably not get as much use out of the extra time, but my work is varied enough now that it's not an issue.

I did it many moons ago (in the US even!). My role was largely operational at the time so it was pretty much just close more tickets. I kept telling myself that I worked ten hour days anyway so this was just one extra day off, but it really was more tiring. I still liked it.