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

Return flights

03 Sep - 10 Sep	Aer Lingus
01 Sep - 08 Sep	Icelandair
03 Sep - 11 Sep	Icelandair
03 Sep - 13 Sep	Icelandair
03 Sep - 08 Sep	Icelandair
30 Sep - 07 Oct	WestJet
23 Sep - 30 Sep	Lufthansa
29 Sep - 06 Oct	Icelandair
15 Sep - 22 Sep	Icelandair
14 Sep - 23 Sep	Alaska Airlines
05 Sep - 08 Sep	Scandinavian Airlines
23 Sep - 07 Oct	Turkish Airlines
10 Sep - 04 Oct	Scandinavian Airlines
14 Sep - 29 Sep	Icelandair
21 Sep - 07 Oct	Scandinavian Airlines
29 Sep - 25 Oct	Icelandair
14 Sep - 19 Oct	Aer Lingus

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