Just to put in my tuppenceworth. I don't need to read any forums about 'madness' in the US as I'm in the US. There is no fear of CP among the people I meet. Just a recognition that a usually, but not always, mild and unpleasant and very occasionally fatal disease can be prevented by a safe and effective vaccine. Anyway, it's a good thing that as a result of vaccines we no longer need to fear infectious diseases like in the past so that even the health risk posed by CP becomes unacceptable.
Vaccines don't 'lessen' the immune system. My children, by having the CP vaccine, have a BETTER immune system and are less likely to get a secondary infection as a result of having had CP. Being ILL weakens your immune system, rather than being vaccinated.
Why do I need 'faith' in my immune system??? Of course it works MOST of the time and with CP, I know my children's immune system will (probably) eventually fight off the virus (although with a risk, albeit small, of severe complications and death). It's not a question of faith. Does this faith in the immune system extend to antibiotics (perhaps an indication of immune system failure??)
In short, I'd do it if I were you OP. The CP vaccine is showing signs of being very safe (certainly more than CP!) and effective with long lasting immunity. In the worst case, once immunity starts to wear off (and you can estimate how long by looking at the countries which pioneered the CP vaccine - so far so good though!), you can get a booster. There's been no sign of shifting to older ages in countries with the vaccine.
Also the benefit of being in the UK is that your dc will be exposed to natural boosters all the time when CP does the rounds at their school which should increase the long term effectiveness.
There is a vaccine for shingles available so I don't think you need to be concerned about the elderly, drop in the ocean or not. I understood one of the reasons the NHS is holding off with the CP vaccine is a) cost because it needs to be rolled out together witih the shingles vaccine for the elderly and b) because they want to get the numbers up for the really bad diseases like measles before starting another vaccine campaign. I think that with CP, if you don't get high take-up but enough to dent the virus spreading you'll get CP shifting to older ages which will lead to more complicated cases even if the numbers of cases dropped so I think they want to be more certain that when they roll it out, that there will be higher up take than currently predicted. Personally, I wouldn't want to be held hostage to this kind of thinking and I did seek out the CP vaccine when we lived in the UK.