How long before the flight?
When did BA let you know it was canx?
Did you already have a stopover planned in SIN?

Assuming the flight is in a few days and you had no stopover planned, I think there's little you can do. BA are getting you home in good time on an alternative service, without a downgrade. Little room for complaint or compensation - unless you're arriving late into LHR and depending on the time before flying of the notification.