Day 2 test on return from France, can it double as test required for flight home to US?

Hi all, thanks so much for helping navigate travel in these complicated times.
Four of us are flying to London in late Sept. Four nights in London to visit our daughter, then 3-day trip to Paris, via Eurostar, then back to UK for four more nights.

All together we are looking at 20 Covid tests between the four of us to make this happen. That's a lot of chance to test positive and have trip tripped up -- which is another issue we are grappling with. (We are all vaccinated, but realize we can get it, spread it, and could land in an isolation hotel room, despite trying hard to be careful. )
That important issue aside for now, here are a few logistical questions:
We are wondering if the Day 2 test required by UK for return from France can be used twice -- for day 2 and as the test we need to get on our flight home. The timing works, but is that legal, or even possible? That would eliminate need to buy 4 tests.
Also for start of trip can our daughter there buy our Day 2 tests at a pharmacy or online and hold them for us? I'm assuming we could use scan of proof of her purchase to show when required to offer proof? Maybe that doesn't make sense and it would be better for us to have them mailed to her or just bring along with us?
We also could cut France from itinerary, and stay in UK. (My husband's vote.)

Thank you so much.
R and R