You mean that war where Challenger 1 scored a kill in the longest tank-to-tank engagement in history with its rifled gun, and the associated HESH rounds proved to be a valuable commodity against fortified emplacements? Sure the smoothbore gun proved an ample weapon, but it didn't necessarily prove to be any better than a rifled gun - the UK didn't go through Op. Granby with its eyes closed.
The only reason the UK even looked at smoothbore guns for CR2 was for particular economic concerns regarding domestic ammunition production - concerns that didn't exist when the vehicle was developed because subsequent governments were yet to realise that actually, you need to re-stock ammunition from time to time, and periodicity make improved ones, because the situation in the early 1990s wouldn't last forever.
Fact is now that the UK's economy is worse to the point where the initial cost of re-arming the vehicles eclipses the potential financial benefits of using common NATO rounds in future (in political terms at least). So we've kind of missed the opportunity to do it. We had enough problems making a case for re-arming our ISVs with a 40mm gun, never mind MBTs; a type of vehicle that some people all across NATO think are difficult to justify having in such numbers amongst their armies, to make rearming Challenger in the UK worthwhile.