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

Return flights

14 Sep - 07 Oct	Aer Lingus
23 Sep - 02 Oct	Aer Lingus
10 Sep - 24 Sep	Air France
03 Sep - 11 Sep	Air France
03 Sep - 10 Sep	Air France
23 Sep - 02 Oct	British Airways	direct
30 Sep - 07 Oct	British Airways	direct
14 Sep - 07 Oct	Air France	direct
24 Sep - 01 Oct	British Airways	direct
17 Sep - 02 Oct	Aer Lingus
15 Sep - 02 Oct	Aer Lingus
11 Sep - 25 Sep	Aer Lingus
15 Sep - 01 Oct	Aer Lingus
05 Sep - 26 Sep	Aer Lingus
21 Sep - 29 Sep	Air Canada
20 Sep - 29 Sep	Air Canada
01 Sep - 15 Sep	Delta
27 Sep - 11 Oct	Delta
01 Sep - 15 Sep	Virgin Atlantic	direct
21 Sep - 29 Sep	Air France	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 $480

Return flights