Re Sconhome's comments, which I'm not denying in context, I'd like to point out something else regarding attics.
Installing these new joists to prevent compressing the insulation seems odd, because by then you've done a lot of the work to fully convert the attic.
To avoid replacing these joists in a full conversion later, the joists should be sized to take the floor loadings, not merely to avoid compressing the insulation.
If you refer to the building regs you may find a requirement to install new joists in converted attic spaces to take the additional floor loading.
It seems that the normal 5/6 inch [125/150mm] ceiling joists are not deep enough [=strong enough] to take dead and live loads associated with either heavy storage or living requirements.
I think you also need to raise the bottom face of the new joists above the bottom face of the existing joists to avoid any deflection damaging the existing ceiling.
You may also need to fireproof the existing ceiling/ underside of the new floor if this is required.
If you intent to fully convert the attic at a later stage you should also consider insulating, not between the ceiling joists [the horizontal "floor" of the attic], but between the roof rafters [the sloping part of the attic roof].
This should be done by an experienced contractor and to an approved detail regarding vapour check, insulation and cavity ventilation, adding in additional roof vents if required to ventilate above this
Part F requires a 50mm air gap to be left to make a continuous ventilated cavity on the cold face of the insulation. Some new roofing felts/membranes/papers claim to be able to get around this by being "breathable".
Insulating between/just in front of the rafters will keep the attic space within the warm envelope of the rest of the house.
You have to watch out for cold bridges at the eaves, chimney and pipe penetrations, as well as pockets of dead air above and below rooflights.
There are also other aspects to attic conversion covered in the building regs [means of escape/fireproofing stairs/ upspeccing the fire detection and alarm system, installing fire doors, etc.] not mentioned here.
This is just a brief note so you don't think that by upsizing joists to prevent compressing the insulation you've complied with them by default.
The long and the short of it is that insulating the outer roof envelope may actually be a more cost effective solution in the long run than insulating above the ceiling joists if at some point you intend to fully convert the attic.
If you don't and aren't using it for storage [and so not flooring out or putting things on the insulation to compress it] then you can ignore this.