Londoner
London
Refers to the city of London; originating from London or belonging to London. This adjective is not declined.
Die Londoner U-Bahn wird auch „The Tube“ genannt.
The London Underground is also called 'The Tube'.

London
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London
Refers to the city of London; originating from London or belonging to London. This adjective is not declined.
Die Londoner U-Bahn wird auch „The Tube“ genannt.
The London Underground is also called 'The Tube'.

Adjectives derived from city names are often formed with the ending '-er'. These adjectives are always capitalized. Other examples are 'Berliner' (Berlin), 'Hamburger' (Hamburg), or 'Wiener' (Viennese).
Die Berliner Mauer war ein Symbol des Kalten Krieges.
The Berlin Wall was a symbol of the Cold War.
The adjective 'Londoner' (London) is invariable, so it is never declined. It always keeps the form 'Londoner', no matter what gender, number, or case the noun has.
Wir besuchten ein Londoner Museum.
We visited a London museum.
The word 'Londoner' can also be a noun and then means 'a person from London'. Unlike the adjective, the noun is declined: 'der Londoner' (the male Londoner), 'die Londoner' (the Londoners), 'den Londonern' (to the Londoners).
Ein Londoner hat uns den Weg zum Bahnhof gezeigt.
A Londoner showed us the way to the train station.
English
'englisch' and 'Londoner' can both have something to do with England or the English-speaking world, but they do not mean the same thing. 'englisch' refers to the English language or to England in general. 'Londoner' refers only to the city of London and remains unchanged as an adjective. The words can be confused because London is in England. If you mean the language or the country England, use 'englisch'. If you mean the city of London specifically, 'Londoner' is the right fit.

Im Museum sahen wir alte Fotos von Londoner Straßen.
At the museum, we saw old photos of London streets.
Maria liest jeden Abend einen englischen Artikel im Internet.
Maria reads an English article online every evening.