Well I don't know about the UK system, but the R system is a way of measuring how hard it is for the heat to leak in or out. The higher the R value, the more insulated you are. As you say different materials can have a huge difference in how efficient they are at insulating, so 270 mm of one material may be equivalent to 470 mm of another. The R system is designed to allow you to easily compare them, R50 insulation always insulates exactly as well as any other R50 insulation, regardless of the amount or type of material required to achieve it.
R50 is pretty high on the scale of normal recommendations for the US too. Having more insulation can never hurt, it may just be unnecessary. The more insulation you have the easier it will be to keep the house warm in the winter and cool in the summer. If you live in a climate where you don't need to do either for 95% of the year, then it's probably a waste of money to have any insulation. How much it makes sense for you to have depends on your climate, temperature preferences, and cost of heating / cooling.