Assuming that there is any way of removing the blacklist: I'm pretty sure most UK carriers don't even let you pay off the previous person's bill to clear it.
"Size of an iPhone Mini" is tricky, since if the 64mm width is important you get very few options. Here is
a list from GSMArena's phone finder, where my search criteria were simply "year > 2018, Android, bar form-factor (to eliminate watches from the results), width < 66mm". There aren't many, and the only half-decent ones are old (which is important because you have to consider battery condition: both 3 years use and sitting in a box for 3 years will degrade that, though being used for 3 years is likely to be worse unless you have an extremely battery health conscious user).
Relaxing the width restriction to 68mm
doubles the number of results, though most of them are still lower-end than the Pixel 3. The Pixel 3 itself appear when you loosen the width limit to 70mm, but is also a cm taller than the iPhone 12 Mini: if you want to restrict the height as well you will lose most of the extras you gain by being more tolerant of the width.
I tend to not care about resale value because by the time I replace a phone I don't expect it to have any significant resale value left, no matter who made it. But if you are serious about size-matching the iPhone Mini then you'll be looking at either old or second-hand anyway: when I asked for Android phones of similar height and width to the iPhone 12 Mini released in the last 2 years I got
only 3 results, and all 3 are junk that falls far short of the Pixel 3.
Note that I'm writing as someone who has no interest in large phones, and would regard the iPhone 12 Mini size as perfect (being able to use it one-handed on the go matters to me, watching videos on a phone does not). But there simply aren't any recent decent Android options as small as that.
