Which is why you do need something
with more range than the 122mm or 130mm, silly.
And an 8"/60 firing subcaliber rounds at ERCA Charge Super velocities does that. And you get even more range with rocket-boosted subcalibers.
- If it uses 155mm subcalibers, make it compatible with NATO 155mm in addition to the dedicated long-range stuff. Note that you might have to limit the NATO-compatibility to guided rounds if you are using vertical guns. If it's still in a turret you get to use all the dumb rounds as well.
- If you're doing 5" subcalibers, make it compatible with US 5"/54 and maybe even make a set of 4.5" sabots as well (assuming that the UK still makes 4.5" ammo but not barrels). In this case, you pretty much have to use a turret mount instead of a vertical gun, because there are very few guided 5" or 4.5" projectiles.
Overall, I'd prefer 5" subcalibers because that gives you a
much better length-for-caliber in the same storage volume, which means better range. But 155mm means NATO ammo compatibility.