insulation is not relevant to condensation.
What matters is:
the amount of water vapour in the air
the temperature of the air and the things it touches
Water vapour is lighter than air, so it naturally rises within a house until it either escapes, through ventilation, or finds a cold surface to condense on.
The envelope of your habitable space in the loft conversion must be fully insulated, especially the roof slopes. The Building Regulations approval will include the designers specification of how this will be done, it must be followed. The construction, especially the sloping ceiling, must prevent water vapour passing through (the design might include foil-backed plasterboard with taped joints or some other layer). Holes, especially for downlighters, are very pernicious.
The cold space outside your habitable envelope can be fully ventilated. Probably the roof will be built with a permeable membrane under the tiles which, unlike roofing felt, allows the passage of air and water vapour, so it can escape. Ventilating tiles can be added by the roofer if there is any doubt that this space is generously ventilated.
The habitable space must be well ventilated, for example with vents on at least two sides so there is airflow. Windows should have trickle vents. Some occupants have an aversion to ventilation and delight in closing or blocking vents and windows.
Anything that adds water vapour, especially a shower room, must have additional ventilation, such as a ducted fan. It must vent to the outside world. The approved design will include this, but to ensure that it is actually used, it is necessary to wire it to come on with the light switch and run on for long enough to dry the room. People with an aversion to ventilation will try to defeat this.
Because water vapour rises, any major sources, such as wet washing draped around the home, or unventilated bathrooms, or holes in ceilings below, will cause it to rise into the loft conversion.