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

Return flights

05 Sep - 09 Sep	Frontier Airlines
19 Sep - 26 Sep	Frontier Airlines
03 Sep - 11 Sep	Icelandair
16 Sep - 08 Oct	Icelandair
05 Sep - 18 Sep	Scandinavian Airlines
03 Sep - 12 Sep	Icelandair
30 Sep - 07 Oct	Icelandair
03 Sep - 18 Sep	Icelandair
07 Sep - 12 Sep	Icelandair
14 Sep - 07 Oct	Icelandair
17 Sep - 21 Sep	Alaska Airlines
04 Sep - 12 Sep	Alaska Airlines
09 Sep - 30 Sep	Icelandair
26 Sep - 12 Oct	Icelandair
20 Sep - 17 Oct	Icelandair
15 Sep - 12 Oct	Icelandair
19 Sep - 18 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 $566

Return flights