BTW it does seem like upgrading the onboard storage is not a very complex procedure (for those that can microsolder), if you can get hold of the storage NANDs that are compatible. Even more so for minis it seems that the chips are not soldered on the mainboard but use a proprietary M.2-like board, so if you can get a hold of one with higher capacity, replacing it is very trivial.
Have you looked perhaps at local repair shops offering such services in UK? I will have to upgrade mine in a few years as 128GB storage NANDs that have a maximum of 150 TBW paired with not enough RAM will kill those chips fast.
