A cow ? gives us milk ?.
A and An are used before singular countable noun.
Because egg gives vowel sound.
Articles "A" and "AN" are used with some expressions.
For example:
Make a noise, give a warning, have a pain, have a great education etc.
Little knowledge means almost no k knowledge.
The little knowledge means some knowledge.
He knows something that's why his knowledge proved a boon for him in getting job.
Usually Article the is used for Nationalty.
Like, The Chinese, The Japanese, The Indian
Because five gives consonant sound.
There are mainly three articles in English (A,An, The)
We use an when there is vowel sound.
We use a when there is consonant sound.
Europe gives consonant sound
So, London is a European city is the correct sentence.