LEL 2017 / London - Edinburgh - London Audax

is any lightweight bivvy bag going to be good enough if you're not carrying a sleeping bag as well though? (I don't know, no experience of bivvys). That Rab one Hippy posted does look decent if a bivvy alone is enough.
I'd think you're unlikely to end up in quite the same situation as on BCM - you would have slept at previous control before pushing on if doing it again, I'd hope that was a one-off mistake.