As per Ethel, however - be careful!
Building Regs have charged over the last 10 years and you would have to satisfy current Regs. Of particular issue is the level of insulation in the loft space. 10 years ago, 100 or 150 mm of fibreglass did the trick, but not now. With a loft conversion you may be faced with ripping out the plasterboard to get behind the walls and ceilings to insert more insulation.
The second main issue is the structural integrity of the loft conversion floor - Building Control may want to physically check to see what strengthening was done to joists unless you have the detailed drawings - ask them what 'evidence they would require.
The third main issue is escape in event of fire. Domestic dwellings having a second floor (ie.e those over 4.5m from the ground) require a 30 minute fire barrier corridor between the second floor loft conversion and the hallway/main exit door. Usually this means that every door leading to a habitable room from the landing and hall must be changed to be a fire door and a second layer of plasterboard must be nailed to the first floor ceiling.
Don't wish to frighten you off but on what basis was your assertion that retrospective approval would be easy and cheap?