Suspect 1SL was referring to the level of defined compartmentation and/or elements of the requirement that had equipment defined against it - either that or that 80% of the "weight" had been allocated to specific kit (initial weight estimates are undertaken using regression analysis and indicative sections).
For example you would expect the equipment required in Flyco to be identified by now (and I'm sure it has), but whether the individual equipment cabinets, switches etc have been positioned on an arrangement drawing, the requisite services (power, cooling, comms, DTS) arranged and included on the system drawings and a material procurement list for the compartment produced is another matter. I would suspect that the £300M referred to includes these activities.
I haven't seen any of what I would describe as detailed design drawings / info - but then again I'm not working directly on the project. It is progressing, but I don't believe they're in a position to flash up a whole lot of CADDS5 info for Tx to purchasing or production facilities yet.