Lazy explanation : the British are generally too stupid or arrogant to learn other languages, and therefore are ignorant of most "foreign" humour and think theirs is the best.
Best,
Lazybloke : A Brit who speaks only his native language
Longer explanation: Those that say "British humour is best" would best add "for Brits" because the appreciation of humour is massively influenced by history, language, class, age and various other factors. You can collectively describe these factors as cultural differences. And there are vast cultural differences between countries (see also : Brits hate American chocolate, and vice versa)
Come to think of it, there are vast differences in culture & humour just between the regions, which might explain why Peter Kay's standup humour does very little for me (as a soft Southerner) , whilst those from the regions seem to detest Michael McIntyre!