Yeah, I don't envy the MoD folks on this one. Nor the manufacturers.
Take the US Hummer. When designed, it was intended as a general utility vehicle to go near to, but not on, high lethality, short duration battlefields, with well demarked front lines. In that context, it was acceptable to trade armor for speed, as nothing except a tank would survive an anti-tank round. Air transport was nice, but somewhat optional, as everything would be said and done in the Fulda Gap anyway. The much less numerous tracked Bradley IFV was intended a front line weapon, and weighed 30 tons as a result. It was not intended to patrol anything.
20 years on, because that is how long Agile Development takes for weapon manufacturing, the threat is very different. Speed is nice, but those vehicles are never really in a safe zone and they can't out speed the enemy. There are no front lines, just patrol areas. Hummers are not a happy ride, despite being really rather tough for an upscale Jeep. Air transport considerations are paramount. Not to mention that not all rickety Afghan bridges can support 30+ ton vehicles. Tracks are a liability, as they wear out quickly and tear out roads.
There are some vehicles on the market with good survivability, such as the Stryker/LAV and some South African mine-resistant vehicle I don't remember the name of. Perhaps what the MoD and the US DoD need to do is to buy a vehicle for this war, meaning cheap (so that everyone gets one), slow, well-armored against mines and IEDs, not tanks. Now, off-the-shelf.
Rather than a super-vehicle meant for this war and the next one with the godless Chinese in 2032 ;-)