Hi big.al,
I'll dig out the definitive document references (CAA), but in the meantime think of it as an IR with restrictions. (The IR "rights" are easier to find on CAA website)
Restrictions compared to IR:
UK only, including IOM, Channel Isles
DH/MDH are higher (absolute minimums 500' precision/600' non-precision QFE)
No class A airspace (TMAs and ALL UK airways)
Benefits compared to vanilla:
IFR in upto class D (not A, B, C)
VFR above the cloud layer (brilliant - worth it for this alone)
SVFR minima lowered to 3km from 10km
Makes you more "aware"
CAA only require 10 hours post PPL for rating issue. (Plus course and exam passes
)
HTH for starters --> read Thom book 5 that has a load of info.