P.S. The best bit of evidence I've managed to find is this article from New Civil Engineer last year, which says that Phase 1 won't be ballasted, but Phase 2 will, for the reasons given below.
Speaking at New Civil Engineer’s Future Tech Forum Mark Morris, High Speed 2’s director of asset management, railway operations said that the first London to Birmingham phase of the £32bn rail line was set to be built using a concrete slabtrack system, with a ballasted track bed set to be favoured for phase 2 from Birmingham to Leeds and Manchester.
“Phase 1 is looking like it’s going to be slab, with a decision in November,” he said. “Phase two looks like it’s going to be ballast.”
[...]
The first phase of High Speed 2 arguably lends itself to slab track as a high proportion of the route is in tunnel – where slabtrack has already been specified as the trackbed of choice. This is a simple economic decision as slabtrack systems require a shallower base which means the tunnels can be smaller in internal diameter.