You will find more statistics at Statista
If you're looking at the UK only, then the market share is still dominated by Android but not as steeply. This is my source for the UK data for the chart below and I'm guessing their source will be Gartner or similar.
If you look at either chart, you can see that Google is currently winning the battle both at a Global level and in the UK. Whether they've won the war or not is a different matter in different countries. There is still much to play for and it's a dynamic market. Nokia was once as dominant as Google is now.
Understanding your marketHowever, it is also important to note that when you're running mobile activity that you understand the nuances of the market(s) in which you're operating and the habits of your customers. And it's not just an Android vs everyone else story. There are so many shades of Android and such a wide variety of handset capabilities within Android that you also need to understand that at a deeper level. There's everything from the high-end devices from Samsung, Sony Experia and their ilk down to the sub £100 handsets to even cheaper devices in developing countries.
It is still important to avoid memory and RAM hungry apps if you can as many of your customers simply won't be able to access them or if they do, it's at the expense of other apps - a one-in and one-out scenario.
I'm also musing if the first chart will do a full circle and go back to the multiple OS environment we had 7 years ago. Probably not anytime soon, but it could happen. Look at Nokia.