I don't understand why 3G on an iPad is like £15 a month
Because enough people are willing to pay that price, apparently. Sometimes, prices are driven by high demand and, often (always, maybe), companies charge what the market will bear.
I might be wrong, but the iPad is just a iPhone for giants, so actually the data transferred to the phone and the iPad would nearly be same.
I suspect it is a bit more complicated than that. I, for one, am more likely to stream video on an iPad than on a small-screen phone, for example (otherwise, why whould I have even purchased an iPad given that I have an iPhone?). While the capabilities are similar, I expect the difference in size tends to suggest different uses, more convenient usage experience, and increased data.
By mandating GSM networks, the EU assures that consumers can purchase a phone and move an account easily from one carrier to another.
I suspect, also, that this is a factor regarding why the EU is falling behind in 4G network adoption.
In my area, at least, there are many options for those who want to purchase their phone outright and pay less expensive rates for data, text, and voice. I am thinking clearwire, virgin, Tmobile pay-as-you-go, and several others that I cannot remember right now.
I also suspect that the population density differences between Europe and the US is a factor in pricing. It seems to me that it is cheaper on a per-capita basis to provide data to dense cities and towns compared to the vast suburbs and large rural areas found in the US.