Exactly the same boat here mate. I have an Xbox One S that I primarily got for playing 4K blu-rays as it was far cheaper than buying a player.
From what I have read PQ wise there isn’t going to be a huge difference, apparently the Xbox does a good job in comparison with standalone players. I think the main benefit will be the proper remote and less noise. I’m still not sure if that’s worth the £99 though as my Xbox hardly gets touched apart from playing 4K’s. Not sure what to do mate!