Central London now has residential prices per sq foot from about £1K-£5k+ ($1.5K-$7.6K+) - roof space is often the most expensive (read penthouse) and suitable

Central London now has residential prices per sq foot from about £1K-£5k+ ($1.5K-$7.6K+) - roof space is often the most expensive (read penthouse) and suitable roofs sparse (at least in London). How are you planning to find suitable space / scale in such conditions?