Do UK really needs tanks anyway? They are protected by big US ally from large scale real wars, and would be in foreseeable future. For other tasks like expeditionary operations around the globe, only small amount of tanks are required. So, decision to make an upgraded version of existing tank is pretty reasonable.
>It has a gun that everybody else has been using for a while now and it is an updated version of a reasonably old platform yet they say this is future proof.
Gun may be swappable if tank size allows it. For example, initial version of T-62 had 100mm gun, but then it was replaced by 115mm. Technically even turret may be replaced in future if powertrain allows raising the weight limit.
Using same gun as everyone allows using old ammunition that was made in millions from Cold War, and also simplifies service.
At the late of Cold War most projects already included 152/155mm as future-proof gun, but looks like it is far future even today. But it is very strange that western tanks still has trouble with autoloaders.