A couple of weeks ago we got 4 deals via mobiles.co.uk
It was a Black Friday special and we got the iPhone X for £450 upfront and then £22.99 per month for 24 months.
It is a large amount up front, but the total contract price is £1002, which was only £3 more then the phone alone from Apple.
I was slightly sceptical, but all phones have turned up, and are fully working on 02. They are only 12GB data, but as most of the times the phones are on WiFi, we find that is fine!
Now that deal is no longer showing on the website, but we decided on a 5th phone last week, and called them directly. And they honoured the price.
Might be worth a try, it is a fantastic price!