Bob Mannix wrote

Even if there were no insulation there in the first place, the requirements
for 30 minute fire protection between the old house and the new loft
conversion would require new mineral fibre insulation beneath the floor of
the loft conversion, nailed to the joists. To remove it would be highly
irresponsible.

You are right about the ceiling needing to be 30 minute FR, but this is achieved
by the plasterboard and skim (or by lath & plaster in older property). Thermal
insulation material laid above the ceiling adds nothing to the ceiling's fire
resistance. As I said, the original insulation to the area of the new loft
floor should have been removed - this is not needed as there will be new
insulation on/between the rafters. The original insulation in the eaves should
remain unless the new rafter insulation is carried on down the slope to joist
level.

Peter