I've done lots of reading on it and spoken to a number of GPs and this is what I've surmised:
They don't give the chicken pox vaccine routinely in the UK because there isn't yet an effective and affordable shingles vaccine.
People that have had chicken pox itself rather than the vaccine have varying levels of immunity (depending on the severity of the chicken pox they had) and those with weaker immunity especially require frequent re-exposure to chicken pox to boost their immunity against shingles. I think the worry is that if the younger generation stops passing chicken pox around, the older generations will have an issue with shingles.
It's a sacrifice the smaller group for the larger type idea. There are a lot of us out there that had chicken pox itself rather than the vaccine.
From what I've heard, the vaccine gives a high level of immunity against both chicken pox and shingles.
I'm very tempted to pay to get my child the chicken pox vaccine as I think it improves all outcomes for the vaccine recipient. I wish it were given routinely in the UK but I can understand why the nhs has to look at the big picture.