Russell is being humorous. He is using both sarcasm and dry humor in that very special way that British humor works (British spelling: humour).
The explanations below are as good as any for these characteristics of British humor.
Sarcasm is the use of irony to convey. It’s often a sharp, bitter or cutting remark manifest chiefly in vocal expression. This means it’s often the tone of voice which can imply whether something is sarcastic.Professionals in psychology and related fields have long looked upon sarcasm negatively, particularly noting that sarcasm tends to be a maladaptive coping mechanism for those with unresolved anger or frustrations. Psychologist Clifford N. Lazarus describes sarcasm as “hostility disguised as humor”. While an occasional sarcastic comment may enliven a conversation, Lazarus suggests that too frequent use of sarcasm tends to “overwhelm the emotional flavor of any conversation”.
British Humour is quite often referred to as ‘dry’, synonymous to the description of deadpan. This describes a humour based in expressing very little, if anything at all. Humour where there is a tension around whether or not it’s okay to laugh. Or where a situation is so uncomfortable that laughter is the only escape from the tension. Dry humour is often intellectual and speaks of bigger topics in a simplified way. For more dry humour check out Steven Wright with jokes such as – ‘It’s a small world. But I wouldn’t want to paint it…’
Blog from an English-teaching school re British humor
The term conventionality means "the quality of being traditional and ordinary or a part of something that is like this: The artists were drawn to one another by their rejection of conventionality."
So, the philosopher is suggesting two things:
- The British are traditionalists.
- The British have a sense of humor.
That is very comical in fact. It is precisely the British sense of
conventionality that makes it possible for British humor to have
flourished so wonderfully. It is easy for the British to make fun of themselves as they are precisely "so conventional". In every day life, the British are not
known for their sense of humor. They are known, as it were, for being conventional. The perfect kitchen for cooking up batches of funniness, which they have done wonderfully over the centuries.
British humor has a long tradition (going back a long way at least to Piers Plowman where the narrator's name Will mirrors the author's name and the characters' names are quite funny) which is still very prevalent today, one need think no further than Ricky Gervais in the TV comedy, The Office, to see how this dry humor is still very active in the culture. Of course, one needn't forget about Elizabethan Theatre (Shakespeare, Marlowe and Jonson), Restoration Comedy down to the late Victorian comedy of Oscar Wilde's "The Importance of Being Earnest". One cannot overlook the theatrical form of the Victorian music hall, but which existed before that and included the origins of what is today known as stand-up comedy. Of course, even this sketchy (joke) overview of British humour would be sorely lacking without a reference to Monty Python's Dead Parrot Sketch (dead parrot_ENJOY1), a masterpiece of British dry humor and sarcasm.
(Cambridge Dictionary, online)
The point is not that conventionality itself constitutes a sense of humor. It is that it is the basis for British humor. The Wikipedia entry gives a pretty good overview of British humor. British humour