Within each constituency there's a reasonable degree of proportionality, especially in the larger ones, such as the five-seat constituencies. In three-seat ones, in particular, you might not get such proportional results, but what nearly always happens is that, simply on the law of averages, if a party loses out in one place they'll win out on another occasion. Our third party, for example, the Labour Party, might win 10% of the votes on average in most constituencies and sometimes that's enough for a seat, sometimes it's not enough for a seat. On the law of averages, they end up with something like 10% of the seats, generally. It would be very unlikely, statistically, that they would systematically lose out nearly everywhere. In practice, it does deliver quite proportional results, as you say.
On July 26th, 2016. See this statement in context.