AirHint found the following cheapest return flights from Seattle (SEA) to London (STN) in September 2025 (see full year).

Return flights

16 Sep - 23 Sep	Icelandair
08 Sep - 24 Sep	Icelandair
05 Sep - 18 Sep	Icelandair
13 Sep - 21 Sep	Icelandair
09 Sep - 20 Sep	Icelandair
20 Sep - 28 Sep	Icelandair
03 Sep - 12 Sep	Icelandair
24 Sep - 29 Sep	Icelandair
17 Sep - 22 Sep	Icelandair
12 Sep - 22 Sep	Icelandair
11 Sep - 16 Oct	Icelandair

Also check the deals for one-way flights from Seattle to London in September.

The deals are updated regularly. The prices may go up as tickets are purchased by other travellers. Only the cheapest flights are offered. Select a price to book and secure these cheap prices.

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Cheapest return flights from Seattle to London in September from $492

Return flights