All Xiaomi phones on any network seem to be unlocked. Most of the Chinese manufacturers supply them like this.
There are hundreds of pages of posts on the boards.ie Bargain Alerts site on getting discounts for unlocked phones when switching with burner sims to avoid changing number, when discounts are offered based on porting. In most cases these are with Vodafone, which has perhaps the worst e-commerce site in terms of reliability known to man, but works equally well with Three etc. when they have deals on these Chinese phones.
I've also seen them cheaper elsewhere online (not all the way from China - e.g. AliExpress, BangGood etc.) but haven't used any of the stores in question and can't vouch for them so won't post links.