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

Return flights

21 Sep - 28 Sep	Icelandair
05 Sep - 14 Sep	Icelandair
11 Sep - 20 Sep	Icelandair
13 Sep - 20 Sep	Icelandair
03 Sep - 10 Sep	KLM
04 Sep - 11 Sep	Icelandair
10 Sep - 17 Sep	Icelandair
16 Sep - 23 Sep	Icelandair
19 Sep - 26 Sep	Icelandair
13 Sep - 22 Sep	Icelandair
26 Sep - 03 Oct	Icelandair
25 Sep - 08 Oct	Icelandair
29 Sep - 18 Oct	Icelandair
26 Sep - 05 Oct	Icelandair
28 Sep - 05 Oct	British Airways	direct
26 Sep - 03 Oct	Air France	direct
29 Sep - 18 Oct	Virgin Atlantic	direct
25 Sep - 02 Oct	Icelandair
26 Sep - 05 Oct	Air France	direct
25 Sep - 02 Oct	British Airways	direct

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 415€

Return flights