What was the inspiration behind Philip Larkin's Poem 'MCMXIV'?

The Poem:

Those long uneven lines

Standing as patiently

As if they were stretched outside

The Oval or Villa Park,

The crowns of hats, the sun

On moustached archaic faces

Grinning as if it were all

An August Bank Holiday lark;

And the shut shops, the bleached

Established names on the sunblinds,

The farthings and sovereigns,

And dark-clothed children at play

Called after kings and queens,

The tin advertisements

For cocoa and twist, and the pubs

Wide open all day;

And the countryside not caring

The place-names all hazed over

With flowering grasses, and fields

Shadowing Domesday lines

Under wheats' restless silence;

The differently-dressed servants

With tiny rooms in huge houses,

The dust behind limousines;

Never such innocence,

Never before or since,

As changed itself to past

Without a word--the men

Leaving the gardens tidy,

The thousands of marriages

Lasting a little while longer:

Never such innocence again.

Why did he write it?

And what empowered him to write it?