Looking to improve acoustic and thermal performance before relaying wooden floor
If you want to reduce the sound of footsteps, you should use a soundproofing underlayment (which is essentially a sheet of rubber).

If you want to reduce other noises in the rooms, such as voices, TV, etc., then the insulation is a good choice. You should use enough to fill the cavity; if the cavity is 100mm thick or more, then the 100mm loft insulation should provide significantly more sound insulation. The actual difference in performance between regular building materials and their "soundproof" counterparts at equivalent thicknesses are typically quite small, making the added cost difficult to justify. Specialized construction techniques are much more important than specific materials for achieving overall good soundproofing (in other words, the difference between no insulation and insulation is much bigger than the difference between normal insulation and special insulation).

As far as a thermal barrier, if you're heating/cooling both levels, then you're not likely to benefit much from it (and it could even reduce comfort by allowing some rooms to get too hot/cold).