Banjo:
The accident report you refer to is available
from the AAIB website as a PDF document.
From the report,
The Terminal Area Forecast (TAF) for Biggin Hill for the period between 0700 hrs and 1600 hrs on 19 July 2003 gave a surface wind of 190°/10 kt with visibility in excess of 10 km and scattered cloud at 3,000 feet agl. At 0820 hrs an actual weather observation at Biggin Hill Airport recorded a surface wind of 160°/08 kt, varying in direction between 130° and 210°, with visibility in excess of 10 km and no cloud below 5,000 feet agl. The temperature was 22°C and the dew point was 16°C. In general the weather was fine, typical of a good summer's day, and it remained so for the duration of G-CSPJ's flight.
so I'm not convinced inadvertent IMC is relevant here, but you do make a good point about high rates of descent achievable under certain conditions.