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

Return flights

01 Dec - 09 Dec	Icelandair
01 Dec - 10 Dec	Icelandair
01 Dec - 11 Dec	WestJet
19 Dec - 26 Dec	Icelandair
14 Dec - 29 Dec	Aer Lingus
12 Dec - 04 Jan	Aer Lingus
28 Dec - 13 Jan	Aer Lingus
02 Dec - 16 Dec	Air France
20 Dec - 28 Dec	Icelandair
30 Dec - 07 Jan	Air France
04 Dec - 11 Dec	Air France
25 Dec - 04 Jan	Air France
26 Dec - 02 Jan	Aer Lingus
19 Dec - 29 Dec	Aer Lingus
19 Dec - 02 Jan	Aer Lingus

Also check the deals for one-way flights from Seattle to London in December.

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 December from 481€

Return flights