They use Chobham+ (Renford Armour from memory?)
The problem is that that the UK doesn't have the lift ability that the US does in a crisis. If we get involved in a general conflict we will be on the side of the USA unless something really, really odd happens. If we get in a regional one it is likely that the US will intervene. If for some reason, like Timor Leste, the US is busy elsewhere they will still have lift available to ship stuff overseas. Finally, if the US can't lift it, no one is capable and we're well and truly stuffed
The big problem, in my uneducated but opinionated opinion, with the M1 is the mileage. We haven't got the ability to fuel the beasts with as much fuel as they need in a theatre sense. We run the risk of a smart enemy, and if you bank on your enemy being dumb you're already half way to losing, a smart enemy will strike at our under developed strategic fuel transport system and not have to worry about fighting the actual tanks. The US and other first order combatants don't have to worry about that, they can take losses in their strategic logistics and still win a war. We don't have that option.