I'm not religious but evensong at Westminster Abbey was pretty darn memorable for me regardless of denomination. You can get details when you visit there during the day, I think they open up for it around 5ish and only take a few dozen people.
If you're a real Canadian
, you'll also want to go visit the Vimy Memorial. Information and how to get there is here:
http://www.veterans.gc.ca/eng/sub.cf...ls/ww1mem/vimy