主谓一致The number and diversity of British newspaper is conside

这里 The number and diversity（数量和多样性）
被看成是一个整体,所以谓语用第三人称单数.这里取决于那个the.如果把主语改成The number and the diversity of
British,那么谓语就应该是用are.再举个例子：
The writer and musician is delivering a speech,这里的The writer and musician是同一个人,意思是这个人即是作家也是音乐家,所以谓语用单数.
如果改成这样
The writer and the musician are delivering a speech,这里有2个the修饰,证明作家和音乐家是两个不同的人,所以谓语用复数.