Sydney has fairly mild weather with summers reaching about 26°C and winters averaging about 10°C, although the nights can get a bit chilly. The height of the summer is in January whilst June to August are the coldest months. Rainfall is spread fairly evenly throughout the year with around 12 rainy days per month each month - so you're likely to get a mix of sunshine and showers whenever you go.
Summer is the peak time for holidays to Sydney which can see busy crowds both at the attractions and at the beaches, and is especially popular with us Brits who are more than happy to leave the winter behind. To get the best of the weather whilst taking advantage of cheap flights and accommodation, spring and autumn are ideal. If you prefer the colder weather then escaping the UK summer for the temperature Sydney is a great idea, the sights will be emptier, and you can take advantage of the snowfall in the nearby Snowy Mountains too.