Icelandair is showing 2-in-1 trips that allow you to visit both London, UK and Iceland for $402 (carry-on only) to $522 (checked bag) roundtrip including taxes.

You can also book these as just a regular roundtrip flight to London, but there will be a 26 hour layover in Iceland on the way home (see instructions below). 
 

Availability for travel

late October, early November, first half of December 2019

January, February, March 2020
 

How to find and book this deal

1. Go to the Icelandair website

2. Enter London (LON) as your destination

3. On the next page, select the option 'Icelandair Stopover'
 

- Search for a flight like this one:

Vancouver (YVR) to London (LON)
London (LON) to Reykjavik, Iceland (KEF)
Reykjavik, Iceland (KEF) to Vancouver (YVR)
 

Try one of the following date combinations:

Oct 27 to Nov 4 to Nov 7
Oct 28 to Nov 4 to Nov 7

Nov 10 to Nov 18 to Nov 21 
Nov 17 to Nov 25 to Nov 28

Dec 1 to Dec 9 to Dec 12

[2020]

Jan 12 to Jan 20 to Jan 23
Jan 13 to Jan 20 to Jan 23
Jan 19 to Jan 27 to Jan 30
Jan 20 to Jan 27 to Jan 30

Jan 26 to Feb 3 to Feb 6
Jan 27 to Feb 3 to Feb 6

Feb 2 to Feb 10 to Feb 13
Feb 3 to Feb 10 to Feb 13
Feb 16 to Feb 24 to Feb 27
Feb 17 to Feb 24 to Feb 27

Mar 8 to Mar 16 to Mar 19
Mar 9 to Mar 16 to Mar 19
 

dates for $453 (carry-on) to $573 (checked bag)

Jan 10 to Jan 18 to Jan 21
Jan 12 to Jan 19 to Jan 21
Jan 17 to Jan 25 to Jan 28

Jan 19 to Jan 26 to Jan 28
Jan 24 to Feb 1 to Feb 4
Jan 26 to Feb 2 to Feb 4
Jan 31 to Feb 8 to Feb 11

Feb 14 to Feb 22 to Feb 25
Feb 16 to Feb 23 to Feb 25
Feb 21 to Feb 29 to Mar 3

Mar 1 to Mar 8 to Mar 10
Mar 6 to Mar 14 to Mar 17
Mar 8 to Mar 15 to Mar 17
 

dates for $533 (carry-on) to $653 (checked bag)

Oct 16 to Oct 23 to Oct 26

Nov 6 to Nov 13 to Nov 16
Nov 8 to Nov 16 to Nov 19
Nov 13 to Nov 20 to Nov 23

Nov 15 to Nov 23 to Nov 26
Nov 27 to Dec 4 to Dec 7
Nov 29 to Dec 7 to Dec 10

Jan 8 to Jan 15 to Jan 18
Jan 15 to Jan 22 to Jan 25
Jan 22 to Jan 29 to Feb 1
Jan 29 to Feb 5 to Feb 8

Feb 7 to Feb 15 to Feb 18
Feb 19 to Feb 26 to Feb 29
Feb 28 to Mar 7 to Mar 10
 

screenshot from the Icelandair website

You can also book these as just a regular roundtrip flight from Vancouver to London and back, but there will be a 26 hour layover in Iceland on the way home

1. Start with a Google Flights search like this one:

Google Flights: Vancouver to London (LON)

2. Click on the departure date box to open up the calendar view and browse for cheap date combinations.
 

- example dates:

Oct 16 to Oct 23, 30
Oct 28 to Nov 4, 6, 13
Oct 30 to Nov 4, 6, 13

Nov 6 to Nov 13, 18, 20

[2020]

Jan 8 to Jan 13, 15, 22
Jan 13 to Jan 20, 27, 29
Jan 20 to Jan 27, Feb 3, 5

Jan 27 to Feb 3, 10, 12
Jan 29 to Feb 3, 5, 12

Feb 3 to Feb 10, 12, 19
Feb 5 to Feb 10, 12, 19
Feb 12 to Feb 19, 24, 26

Mar 2 to Mar 9, 16, 18
Mar 9 to Mar 16, 18
 

screenshot from Google Flights: