Maybe it's the load bearing capability of the taxiways rather than their dimensions.
Wiki has the MTOW of the Shack at 86000 lbs, the P-8 more than twice as much at 189000 lbs.
Or yet another iteration - perhaps its new taxiways to new parking for them.
It does seem a bit odd to invest 400 million in an MPA base when as you say there's the same thing up the road. What's more, the Royal Engineers are closing Rock Bks (i.e. Woodbridge) and it's an Engineer Regiment in Kinloss.
Maybe the 400m is outweighed by the "through-life costs" of having 3 sites instead of 2.