Bluebottle (Calliphora vomitoria)
There are several very similar species of Bluebottle and I cannot be absolutely certain that this is Calliphora vomitoria but, as that is the most common it possibly is.
Vomitoria says it all really! The females are attracted to fish and meat for egg laying and houses are a pretty good place to find such things. They also lay on carrion outside of houses too of course.
They are around most of the year, whenever the weather is mild.