What do you have at the moment?
I ask this because the notion of an "upgrade" is utter bollox and has been since around the advent of the iphone 6/ Galaxy S6.
Phones since those are perfectly capable and functional and any newer phone won't be doing much, if anything better. You are simply paying for a (unnecessary) new phone.
If your phone is anything newer (providing it isn't a bargain basement phone) then i suggest you just keep it come of contract and go PAYG. the rates for calls and data are much better.
I am now PAYG and pay 20 quid a month for unlimited data and i regularly only spend about 5 of those quids that i have topped up with leaving me with around 15 carried forward.
When my phone does eventually die, i will have a tidy sum carried forward for a deposit on a new one.
If you do have a shit phone and just need a new one, meh, pick your poison, or in the case of the iphone, bend over and take it.....