Does loft conversion need air gap?

Yes it is important to have a air gap between insulation and boards.

It's important to leave a ventilated air gap between the insulation and the boards to prevent condensation on the underside of the boards.

To reduce the amount of condensation build up in the roof to a minimum. Provide a minimum 50mm air gap above the insulation material to allow air circulation

Under the loft floor, you don’t need a ventilated air gap — you need sufficient insulation and compliance with Part L (thermal performance) and Part E (sound insulation).

Standard build-up is:
• Acoustic/thermal insulation between joists (typically 100–150mm mineral wool)
• Structural floor deck over the top
• No air gap is required

The only place an air gap is normally relevant is behind the roof tiles, where a ventilation path is needed unless the roof uses breathable membranes. The floor zone, however, is insulated solidly.

No not under the floor but a 50mm air gap required in the rafters

What all loft conversions need is the correct size joist to span the distance it has to span across normally 200mmx 47 mm but you bc will need to get this approved by building control also not forgetting any structural alters will need structural engineers calculations and a building inspectors approval.
there is always a gap between the plasterboard and flooring above we also put insulation in the gap . you only need a air gap in the sparse on the roof to prevent any destroy from forming