Now that the "Combat Package" of the Challengers is being added via the Challenger Mk.3, I think its time both the Challenger 1 and Challenger 2 get their hull updated. This would make their lower front plate not HEable by every gun in the era, as well as making it autocannon resistant.
The Challenger 2 in the picture is from September 2000, but it would have been developed before that date, and it is the same armor kit as the Challenger Mk.3.