if i buy an apple XR phone in Australia can i use it on a UK network

Looking at tech specs, Apple lists the iPhone XR model A2015 on both their UK and Australian website. So it looks like the same phone will be sold in both countries, supporting identical LTE bands.

Apple publishes LTE support here: https://www.apple.com/iphone/LTE/, however, the details for iPhone XR are as yet unlisted. It would be wise to check this link again before buying.

Also, you should ensure that the phone is carrier unlocked if you want to use a UK sim card.

Also note that if you do purchase a phone in Australia that you will not have any warranty coverage in the UK. You would have to return it to Australia for any warranty service.