There's quite a lot of species of Coprosma. Wikipedia says 45 in NZ, and about another 45 in warmer places. I was in the Savill Garden (in Windsor Great Park) this weekend, which has a large NZ garden. The planting was chosen as intended to be hardy in the UK. It should have suffered a low in the vicinity of about -9 this winter, I guess, as it is well away from dense urban-ness, and that is the typical minimum that other people I know a few miles outside the M25 and well inland suffered. There were one or two gaps in the planting where plants had died over the past winter, but very few overall. There were several Coprosmas which looked untouched, and one or two other that looked like they might have suffered a bit, but were still alive. But probably not the most decorative kinds that sell well. And in general the garden did include several things that are not commonly cultivated, as they are not normally considered particularly ornamental.

So I reckon there are some pretty hardy coprosmas available, but they may not be the most commonly cultivated.

It is not an uncommon situation. For example I have Metrosideros umbellata, the southern rata, the hardiest of the New Zealand "Christmas trees", very little seen in Britain even though it is a lovely thing and pretty hardy. You'll find it much easier to buy a Metrosideros robusta, which can only really be grown in very mild corners of Britain. M. umbellata survived this past winter untouched with me in the Chilterns. I don't even have it in a very clever place - I put it in a large pot rather than in the ground, as I once intended that I might drag it into the garage if cold weather threatened. But it's too heavy to move, so it stays there, not even close to the house.