I usually just leave them. The time expended on redeeming them isn't worth the return. I tend to be dead before they get so bad as to need a scroll bar, anyway.
I thought Roger's idea is best. It's what you can do now with endowment policies etc - sell them for a fraction of what you've put in (to realise the capital). The broker pays off the remaining term and makes their profit on maturity.
I think the percentage paid should vary - with ones that are a system or two away from the point of sale being (say) 40% while those that are furthest away yield 10% (A quick spreadsheet look gives the formula
=if(40-40*A2/100<10,10,40-40*A2/100)
where A2 is the distance from the bounty system in ly