The Queen and Philip are both now very old, as you get older the flu becomes more and more serious. It is right to take strong precautions in such a situation. The newspapers will inevitably make up stories but there is nothing to suggest there were other reasons for them delaying going to Sandringham.
Queen Elizabeth II and Prince Philip have resumed their holiday travel to Sandringham despit having heavy colds. Of course this is the real reason behind their trip being postponed. There are an absolutely huge amount of colds and flu viruses going around Britain at the moment, the chances of that happening to them are very hig.h.
Heavy colds for older people can often be quite debilitating and certainly could mean that they would want to postpone their travel plans. The fact that they decided to leave only 24 hours later suggests that they were battling the effects of a minor illness. There doesn't seem to be any evidence that anything else was going on to make them postpone their trip.
Well, in a way I suppose it actually is - the Queen and her husband want to protect their lives and wisely chose to curtail travelling away from the safety of home. Terrorists and just plain crazy people would love to make a statement with such a well-recognized head of state paying the ultimate price. Just look to the recent assasination of Russia's ambassador to Turkey.