Did the SIN - LHR flight deck give a briefing as doors closed? Well, I know they did, what did the pilot say? There are 2 factors here, the reason for the delay getting to LHR and "countable hours". If the reason for the SIN-LHR delay is countable then you may have an argument, but if the delay was non countable then the LHR-MAN is also non countable, since the weather is bad. The gap between 11:00 hrs and 12:30 is not countable. I can't actually see BA giving you compensation here, but the details will help determine if CEDR would do so.