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

Return flights

02 Sep - 16 Sep	Aer Lingus
02 Sep - 26 Sep	Aer Lingus
03 Sep - 26 Sep	Aer Lingus
02 Sep - 17 Sep	Aer Lingus
03 Sep - 11 Sep	Aer Lingus
14 Sep - 26 Sep	Aer Lingus
09 Sep - 16 Sep	Aer Lingus
30 Sep - 21 Oct	Aer Lingus
06 Sep - 13 Sep	Icelandair
01 Sep - 15 Sep	Icelandair
09 Sep - 30 Sep	Delta
30 Sep - 07 Oct	British Airways	direct
24 Sep - 01 Oct	British Airways	direct
30 Sep - 21 Oct	Virgin Atlantic	direct
27 Sep - 11 Oct	Icelandair
11 Sep - 30 Oct	Air France
04 Sep - 30 Oct	Air France
11 Sep - 30 Oct	British Airways	direct
04 Sep - 30 Oct	British Airways	direct
27 Sep - 05 Oct	Delta

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 396€

Return flights