At least 3 articles have appeared this year about the Poetry Prize culture. They're all good reads.
Fiona Moore's Poetry prizes: the elephant on stage blogpost does the stats, looking at the "last ten years of shortlists, from 2004 to 2013, for both the TS Eliot Prize and the Forward Prize for Best Collection". Follow-ups to that post present stats for some smaller competitions, and mention the Poetry Book Society choices.
Joey Connolly's The Glittering Prizes (The Poetry Review, Summer 2014) considers some explanations of those stats
- the biggest publishers publish the best books
- judges have connections with those publishers, and with the poets (who they may have taught)
- the prizes aren't intended to pick the best poetry but to catch the eye of the public
- committees go for safe choices
In isolation each of these factors might be natural enough, and compensated for, but in combination their effects accumulate. He suggests that "more transparency would be a start". In the States they've already taken steps in that direction.
Jon Stone's Five Fixes For Contemporary British Poetry Culture #1: Prize Culture suggests that we could "Acknowledge prize culture for what it is and what it does, and make it do its job better". He also calls for more openness.
Sending the collections over to the States for someone over there to judge might reduce some bias. Or perhaps the UK could try what Italy's RAI did, and give literature the "X Factor" treatment -