For my top term for a period at end May/early June comparing Adwords with Yahoo SM for a very UK specific term.
Yahoo display rate 100% impressions 3176
Adwords display rate 50% impressions 11,979
I would estimate that if I had Adwords on all the time impressions would be around 20,000.
I'm on the second page of Yahoo organic for this term so virtually all of my traffic is paid for. I'm #2 on Google organic.
66% of my traffic comes from Google paid and organic combined.
8% comes from Yahoo.
It is difficult to be definitive with these figures because I don't know what traffic I would get if I was #2 on Yahoo for this term.
Across all terms I would estimate that Yahoo is at around 20% of Google in the UK.
Occasionally I have seen big spikes in traffic when a DC with better rankings than I am seeing on google.co.uk is pointed at a big UK ISP.