When we did a similar thing, we had a structural engineer do the survey (generate a report/drawings etc) and then a builder did the work. The council also inspected once the wall was down and the steel beam was in. We used NN Engineering Consultants Ltd, they're based in Enfield. Our builder was Radek Tober (you'll find plenty of recommendations on here for Radek). Happy to recommend both companies. No problems at all.