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

Return flights

01 Sep - 11 Sep	SWISS
10 Sep - 18 Sep	SWISS
22 Sep - 29 Sep	SWISS
09 Sep - 17 Sep	SWISS
24 Sep - 08 Oct	SWISS
02 Sep - 16 Sep	KLM
08 Sep - 24 Sep	KLM
02 Sep - 09 Sep	Delta
17 Sep - 24 Sep	SWISS
16 Sep - 23 Sep	SWISS
16 Sep - 01 Oct	SWISS
22 Sep - 06 Oct	SWISS
23 Sep - 29 Sep	SWISS
09 Sep - 06 Oct	SWISS
25 Sep - 01 Oct	SWISS
22 Sep - 08 Oct	ITA Airways
03 Sep - 13 Oct	SWISS
27 Sep - 08 Oct	SWISS
27 Sep - 04 Oct	SWISS
09 Sep - 05 Oct	ITA Airways

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.

Book a cheap hotel in London

Cheap car rent in London

Cheapest return flights from Washington to London in September from $541

Return flights