There are a couple of issues here but to answer your specific questions first. It is important to insulate correctly to reduce heat loss and to meet the energy conservation guidelines.
A new layer of insulation will need to be added immediately below the roof covering. This new insulation will normally reduce the available headroom as the insulation will be thicker than the existing rafters possibly by as much as 100mm.
Most properties that have unconverted lofts are likely to have some insulation laid between the existing joists but the building regulations now call for a layer of insulation to be laid on top of the joists.
To reinforce the floor you could sister the existing joists with new timbers to add strength before boarding out the roof space though of course it is always good practice to seek the advice of a structural expert if possible.
One issue you need to be aware of is that by using the roof space as an office, occasional or not, the loft will be deemed to be a habitable room and therefore subject to building regulations approval.
I imagine that this isn?t exactly what you want to hear but the building regulations are very rigorous in how they interpret the use of the loft space. The reason for the building regulations being so rigorous is because of the threat of fire and all habitable rooms must comply with Approved Document B of the building regulations.
Because of this an office will need to have building regulations approval and will need to be accessed by a permanent staircase rather than a loft ladder with fire protected escape routes.
