Charity ride, I'm assuming proper campsites? I'd take a tent.
Probably slightly warmer which for me = better sleep and it's not like a little extra weight is a big issue. No worrying about trying to find trees or stuff to tie off on if you're in a field or have allocated spaces to fill.
Headed up London > Inverness on a charity ride in a couple weeks. Thoughts on a one man tent Vs a bivvi bag + tarp?