Normally this is because there is predicted reduced capacity at LHR due to weather, so airlines are asked to cancel some flights. They will of course choose the short-haul / domestic flights first. No help to you when your flight is one of those affected.
I'm sure somebody will be along with a list of cancelled flights. If yours was the only one, then you may have a case
Edit - Just noticed c-w-s comment right above yours.
Edit 2 - Just noticed the full chain of emails above... Not sure why these didn't show when I clicked the "Next unread" button.
Edit 3 - I give in... God knows what I'm doing. I'm off for a nice brunch.
