100 hours comes from Cap804, which says you need 200 hours for a CPL(A) but can credit 100 hours of helicopter flight time if you hold a CPL(H).
*It also says you need 100 hours PIC, but this can be helicopter time as well - I got a CPL(A) & MEIR issued with less than 70 hours PIC. The only thing you actually need aeroplane PIC for is an MEP rating which I didn't bother with.