As far as I am aware one would need planning permission to raise the height of the roof. Not sure whether there is any leeway. Give Lewisham Planning a call. They may know this. Otherwise the various local societies will have contacts with some knowledge of planning law. You'll also probably find out some background on Google. An interesting fact is that the planning department should be working on your behalf as much as the person having the work done.

If PP is needed but not obtained, it will be up to Lewisham whether it is in the public interest to do anything about it.

Interested if others know better on this matter.