We need the two types of plane, its important to understand the difference.
Originally in the 1960s when we last went through this debate, we picked the C130 (for whatever reason) to tactically move stuff (troops, ammo, light equiptment and even some of the lighter armour we had such APCs etc) around. This was also designed with the core capability of not requiring a runway much more than a dirt track 600-700 metres long and then being able to unload a relatively large amount of kit quickly, then take off again.
The large stuff was then going to go via the navy/commercial shipping. It will always be ridiculous to fly a challenger division out to a warzone. Even with C-17's, they can only take 1 tank each, it will take an age to fly the armour out one tank at a time, plus all the support staff, gear, REME support tanks etc. Not including the fact that we still then need a friendly airport waiting with unloading facilities for a fleet of 60+ ton MBT's.
This difference in role is important, C-17s are good at airlift, and yes they can lift at lot. However they're not going to be lifting tanks, so the weight limit on A400's is irrelevant. The increased capacity does make a difference, but then again, try and land a C-17 on a dirt track and you'll quickly have the answer as to why we need TACTICAL airlift, not just strategic.
And at the end of the day, why don't get just keep hiring the C-17s/Antonov's anyway? Seeing as Lewis is stating that £100m is purchase cost, plus £100m support (plus training and running the actual bases to keep these things in i imagine) It may well be cheaper to pay by the lift.
Maybe FedEx or UPS will start a military shipping service that would commercially challenge the airlift capabilities of the MOD? We do live in a capitalist country after all, free market and all that.....