I heard both US and Helsinki security staff say "anything than can be smeared is classed a liquid" (well in Helsinki, it was a bit more broken than that...but still better than my Finnish!)
In US it was to a women asking about lipstick, in Helsinki it was to a man with some sort of hair product - he unscrewed the lid held is upside down and shook it to demonstrate how non-liquid it was. But not accepted.
Don't know if LHR takes the same rule...or whether "smearable goo" on the inside is counted! This article claims filled doughnuts will be not berejected...but it's journalism not security rules, and 2+ years old, so not gospel.
http://www.theregister.co.uk/2006/12...ughnut_danger/