It all depends on what you want to do and what your attitudes are to the outdoors. You can go at any time of year but outdoor temperatures are most reliably highest in July and August, although because of the location, weather systems and the Gulf Stream, it can be warmer there in December than in England and the warmest day of the year might occur in May or September. Indoor temperatures are always higher than in any British native home, and that is 24/7. The scenery is starkly beautiful, you really can be totally alone with nature if you want to, and the people treat you genuinely as an individual as opposed to just another face to avoid eye contact with. The best part of the country is to be found in the way the society operates, but that you cannot begin to explore on a long weekend. In short, if you want to go to the beach during the day, the pub at night, and live in an impersonal hotel in between, don't waste your time on Iceland - it is not best for you nor would you suit it.