Can I use my iPhone 4 in London and Ireland?

I'm traveling from the US to London and Ireland. Will I be able to use my iPhone 4?

Yes but will be VERY expensive… VERY! We have horror stories, here in the press, all the time… If leave the UK, even for next door inThe Irish Republic or France, costs are eye watering…

Yes, but try to only use Wi-Fi. There are many free Wi-Fi spots around london.

No, we don't have these new fangled 'mobile phones' or 'telephone networks', we use phone booths.