It doesn't particularly blander than other Western European cuisines, yet is a common butt of jokes.
It's difficult to tease apart. Language often is.
Currently it's thought that the (often) American perception was brought back by servicemen after WWI. Subject to submarine warfare and strict rationing, British cuisine went to hell. The story goes that the naming if sausages "bangers" stems from that time, due to adulteration with filler, sausages swelled up until they popped the skin.
This is much the same as the French used to refer to the Brits as "Les Rosbifs". Much of that impression was gained during English (or British) campaigns on the continent. Lacking commissary, foraging yielded food easy-to-cook. Beef is one of the easiest, and large animals feed multiple men.
The common thread seems to be that non-Brits have, to some extent, based opinions on exposure to the diet at its worst, during wartime.
In addition to being enthusiastic adopters of foreign elements (witness curry) the British cuisine has a vey savoury character. Worcester sauce, and all that.
source: Colin Spencer, "British Food".