1 2 3
It is a very common phrase in the UK indeed. I believe it comes from football. When the ball goes in the supporter says 'get in' as a way of expressing delight and relieving of frustration at gaining the upper hand (which could be seen as good luck). This has extended to other areas, when your horse comes in, when you pass an exam etc. I believe personally it is used when you are the favourite or there is a low element of risk. For example if I won the lottery the chances are so unlikely 'get in' probably wouldn't be used, but could be by some.