To be fair - I've never done the drive during the day - it's always been either early or late. I just find it so much quicker getting down to Briomley and through that way rather than the stop start of Norwood Junction/Selhurst and down to A23 near IKEA - and Purley is a PAIN. And as for Coulsdon... Basically, I hate the A23 and will do almost anything to avoid it.
Never been caught on the M25 but I agree - t's always a risk...