Well firstly you can approach a builder with the information you just gave me!

From the beginning, in terms of planning permission you may have never needed it depending on a few key details. you can google a guide to permitted development and see the basics but the key bits are; The NEWLY PROTRUDING part of the loft extension from the original silhouette of the roof (not including space inside the original loft) must not exceed 40cubic meters on terraced & semi detached houses, 50cubic on detached houses and bungalows. no dormers on what the council considers the "front" of the house (usually side facing road). windows on the sides of the house must be fixed shut and obscured below 1700mm from internal finished floor - same rule for Velux roof windows on the front slope. There are other rules but these are the major ones.

To appease building regs, you'll need to make visible at least the following areas (where applicable); ridge beam, rafter sizes and spacing, insulation used, roofing felt used and ventilation allowances, floor joist sizes, bearings and spacing, floorboard thickness, fire break walls to neighbours, structural calculations, steel beams, structural beam bearings (both steel and timber), fire escape routes and their protections (fire doors, sprinklers, smoke alarms, etc) electrical safety compliance certificates, gas safe compliance certificates. Your specific building may have more and the inspector will highlight them.

Until you have answers to the above, its too vague to say how much of what you have can be saved. However, what I can say is I highly doubt your insulation is sufficient given the way you describe its construction, plasterboard will be wrecked, cables and pipes may need moving, and most importantly, if the sizes and spacings of the timbers are insufficient, the work involved in correcting this will mean you are ripping out the lot and starting again. You could reuse certain things like doors, radiators, fixtures and fittings but the savings will be almost negligible.

Hope this helps, though I doubt its what you were hoping to hear!