Very much depends on what part of Europe you're comparing them to. London's summers temperature-wise are pretty similar the majority of years to Berlin, Warsaw, Prague and Moscow, with maybe marginally less sunshine, very similar to Paris only a couple of degrees cooler and with actually less rain, and on average 2-4 degrees cooler than much of western and central Europe north of Italy. I wouldn't call that "very lackluster" in comparison.
Obviously if you're comparing them to those of southern France, the Iberian Penninsula, Italy and the Balkans, then your point is valid.