There are quite a lot of reasons for showing such displeasure, maybe a comment from somebody else at the same time or even just finding a bit of cartilage or bone in the bacon (I myself might very well do such a face if I was eating a bacon sandwich and there as cartilage or a bone in the bacon).

Also it's quite possible that bacon sandwich was simply a bad bacon sandwich.

It's also possible that Ed, being an upper middle class wanker like Blair, had never eaten a bacon sandwich before and didn't like it (which is weird, since even ultra posh upper class English wankers would have been exposed to fried bacon at some point in their lives, if only when eating a Full English Breakfast) or was secretly a vegetarian or vegan.

The very purposeful fake outrage against the Labour Party and Ed Milliband around that created by the British Tabloids during an election campaign has nothing to do with the reality of what was really going on and all to do with the style of British politics and the stupidity of a large fraction of the British electorate (as proven not that many years later by the Leave Referendum campaign and subsequent results).

The picture and the context around it says a lot more about Britons than it says about Ed Milliband.