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

Return flights

30 Sep - 08 Oct	Icelandair
23 Sep - 14 Oct	Icelandair
23 Sep - 09 Oct	Icelandair
23 Sep - 30 Sep	SWISS
29 Sep - 03 Oct	Icelandair
24 Sep - 24 Oct	Icelandair
24 Sep - 09 Oct	Icelandair
20 Sep - 27 Sep	Frontier Airlines
25 Sep - 09 Oct	Icelandair
23 Sep - 13 Oct	Icelandair
22 Sep - 29 Sep	Air Canada
25 Sep - 18 Oct	Turkish Airlines
24 Sep - 11 Oct	Frontier Airlines
20 Sep - 24 Sep	jetBlue
21 Sep - 30 Sep	jetBlue
21 Sep - 24 Sep	jetBlue
20 Sep - 28 Sep	Frontier Airlines

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 $493

Return flights