London: Loft conversion specialist?

Try contacting your local council planning department, they should be able to guide you in the right direction.

"Ben C" wrote

There are many companies that specialise in lofts. While most seem to do the conversion themselves, some do sell components e.g. loft ladders, so might be of help to a DIYer. In any case, it's reasonable to suppose they might have better specialist knowledge than many architects, and they may indeed be able to provide an architect who specialises in loft work.

Following that line of thought I did a Google search on "loft conversions" - and suggest you might do the same. There's interesting stuff there: just for starters I came across this book.