I think that in the Brideshead Revisited era, and particularly among the upper classes, “bore” had gained a wider sense something akin to “no fun”, which would encapsulate dullness and predictability but also being disturbing or aggravating or otherwise forcing people to deal with something they didn’t want to deal with.
Because if you were upper class in that era, fun was the whole point. You didn’t have to work, politics was run in your interest by people you knew who did find it fun, there was an army of servants to shield you from material reality. It was - or should be - tea parties, tennis, the Season, huntin’, shootin’, balls, house parties - whatever you fancied as long as it was diverting.
Anything that breaks up that idyll is “a bore”. Whether that’s someone actually being dull, or bringing down the mood of the party with a big rant, or pater being beastly about one’s allowance - all of these could be “a bore” because all of them disturbed the charming tenor of one’s days.