There is one other possible route to go down. There are two ways of complying with Building Regulations, Full Plans, or a Building Notice.
Naturally, Full Plans needs someone to draw up every little detail. Building Notice needs no drawings. The on-site details are discussed by the builder and the Building Control guy. I use this method a lot, but many builders don't want the added responsibility, so they play safe and ask the client to provide full drawings.
Loft conversions are not terribly "high-tech". The other considerations of insulation, sound etc tend to be more "technical".
If you can't find a builder willing to go with a Building Notice, then think about employing one of the many Designers/Building Technicians who are about.
Full blown Architects do tend to concentrate rather too much on design and aesthetics. That's great for the right project, but sometimes you only need a competent technician.