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

Return flights

03 Sep - 10 Sep	Aer Lingus
28 Sep - 12 Oct	Icelandair
23 Sep - 07 Oct	Aer Lingus
01 Sep - 25 Sep	Icelandair
04 Sep - 21 Sep	Icelandair
14 Sep - 18 Sep	Icelandair
21 Sep - 30 Sep	Icelandair
05 Sep - 12 Sep	Icelandair
11 Sep - 18 Sep	Icelandair
13 Sep - 27 Sep	Icelandair
02 Sep - 14 Sep	Icelandair
11 Sep - 25 Sep	Icelandair
26 Sep - 03 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 397€

Return flights