I'm in the same boat as Steve, Google's got it locked down in the UK and in my experience clients aren't willing to pay for anything other than Google optimisation. However, as you say Steve, good ranking on Yahoo / Bing usually follows success on Google.
One thing I am looking into though is the Yahoo's Shopping search (Product Submit). It seems to be a largely untouched space in the UK and I'd be interested to hear if anyone's seen success with it. From my investigations so far it seems that you have to submit your feed in $s which is obviously a problem for UK merchants, however I see listings in £s. Do we have any UK experts in this field?