Yes, we did stem cell storage with DD a few years back. We used Virgin Health Bank. We chose them because we like the concept of them storing some of the blood for your child to use if needed, but then making some available for donation if needed for others.
There was a thread about this recently on 'Childbirth' and someone on there was very adamant that it's all a load of b*llocks, but personally I think it's worth the gamble of a couple of thousand quid if it might save my child's life in future years. We're going to do the same for DC2, due later this year.
I think there are some hospitals in London which participate in a donation scheme. I saw something on TV about it a few years back, but don't recall the details other than a very grateful stem cell recipient telling their story.
Virgin Health Bank were excellent, really good customer service and the pack was clear to understand. The main arse-ache was arranging the on-call phlebotomist (independent of the hospital) to come and hang around until I'd given birth to take the blood. That cost a few hundred quid.
As it happened, DD got her hands tangled up around her head and I was given an injection of some kind and episiotomy in order to help deliver her after a few hours of pushing. Because I'd had the injection (even though it didn't have time to take effect for DD's birth - the episiotomy did the trick!), I couldn't then deliver the placenta naturally, so that wasn't an issue anyway, and DD was a little distressed, so the cord was cut and done very quickly. I didn't even know anything about the placental delivery. It was whisked away for the phlebotomist to do her thing.
My advice would be that if you can afford it, do it.
HTH.