A lot of people feel the challenger2 is the poorest NATO tank because of the rifled barrel, so is unable to fire the nato standard ammunition instead relying on HESH(High Explosive Squash Head) rounds which aren’t as effective against ERA(they are designed to produce spall inside the target tank(taking out the crew or ammo). What people forget is the Challenger2‘s Gun is the most accurate out there because of the rifled barrel. I’m not certain but I’m sure I read a few pages back that some challenger2’s were fitted with a smoothbore allowing to fire the NATO standard round, the challenger3 will be fitted with a smoothbore.
With reference to the armour I’m pretty sure it’s Chobham armour wasn’t actually applied day to day because of its secrecy and was applied just before deployment, and with Chobham fitted it was pretty much impenetrable
im sure
@steve45 will be along to correct if wrong as tanks aren’t my thing, I’m an Artillery guy(the true kings of the battlefield)

I am by no means an expert myself, all my knowledge of tanks is purely self research and an interest thing for me. I too am Arty guy. Though I am STA not a gun bunny.
There is some merit to the rifled barrel being less accurate when firing APFSDS, you dont want a fin stabilised dart round to spin funny enough, but the L27a1 gets around that by allowing the discarding sabot part spin independently from the dart itself. not ideal but it effectively negates the drawbacks of rifling. The main issue with CR2 gun system is more to do with the 2 part ammunition. it effectively limits the size of the penetrator, where as the NATO standard type is 1 piece and can recess the extra length into the cartridge area allowing for a longer penetrator theoretically improving penetration depth. That said the L27a1 is still a very very effective round and will still have no issue dispatching T90/T80/T72. This will come down to who see's who and can lay on to and fire first.
A lot gets mentioned about HESH, however I suspect this will likely be used to target lighter armour and structures etc, less so for tank on tank, I do question how much Tank on tank there will be at this stage of the war.
The Storage of ammunition I am a little less au fait with. I beleive the charges are stored in a "wet" rack which aids in cook off prevention even when hit.
As for the Chabham armour, it can be found through out and is integral to the design of the tank where its most prevailant and obvious to the eye is the turret, and turret cheeks most obviously. If you ever see a CR2 with it removed you will understand what I mean, it looks like a completely different tank.
I think what you are referring to is the TES (Theatre Entry Standard) Dorchester package, which bolts on further ERA packs to the nose, all down the side of the sides of the skirts and turret. I very much doubt that Ukraine will be supplied with those.
As for the Thermal Imaging system, I'm not so sure on that one myself. I do know that Thales were awarded a contract to upgrade the TI sights about 5 years ago or so. Though I am not sure to what extent this upgrade has been deployed to the tank fleet. I do remember it was quite an improvement over the older system. Therefore I dont know whether or not Ukraine will get the upgraded sights or not.