Well, this is interesting. It's an article from the Economist, showing language diversity in several countries. It gives the probability of two people selected at random from any one country speaking the same language. So in Papua New Guinea, two people are almost certain to speak different languages, whereas in North Korea, they will definitely both speak the same language.
The UK isn't on this list, but the fuller list at Ethnologue gives us a probability value of 0.133, so around the same as Mexico or Australia. However, we have many fewer indigenous languages spoken than either of them (Ethnologue says 12; click here if you want to know what they are) so it looks like we must have more speakers of our minority languages than they do.