Question

主谓一致

The number and diversity of British newspaper is considerable 为什么这里是用is而不用Are

Answer 1

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。。

不明白请追问

Answer 2

In this case, either word works. If there are two items, and each is singular, is can be used as a shortening of the following:
The number of British newspapers, as well as their diversity, is considerable.
If there are more than two items it sounds too awkward. My advice to you is to use "are".

Answer 3

数量和多样性是并列主语的两个方面，所以用单数