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

Return flights

10 Sep - 22 Oct	TAP
10 Sep - 23 Sep	TAP
10 Sep - 30 Oct	TAP
05 Sep - 08 Sep	Scandinavian Airlines
08 Sep - 16 Sep	TAP
16 Sep - 24 Sep	TAP
15 Sep - 29 Sep	TAP
10 Sep - 17 Sep	TAP
18 Sep - 25 Sep	TAP
23 Sep - 30 Sep	TAP
16 Sep - 23 Sep	TAP
04 Sep - 15 Sep	TAP
30 Sep - 07 Oct	TAP
23 Sep - 02 Oct	TAP
24 Sep - 07 Oct	TAP
10 Sep - 14 Oct	TAP
29 Sep - 13 Oct	TAP
30 Sep - 14 Oct	TAP
22 Sep - 08 Oct	Lufthansa
10 Sep - 22 Oct	Virgin Atlantic	direct

Also check the deals for one-way flights from Washington 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 Washington to London in September from 350€

Return flights