Longstaff Cycles in the UK is probably your best bet for a lightweight, touring-style upright trike, it their house speciality. Bob Jackson of Leeds are another frameshop making lightweight trikes.
Theses could not be further from the industrial US style of trike. They are brazed or welded from racing/touring grade butted steel (traditionally Reynolds 531) so are not overly heavy and are typically furnished with lightweight racing wheels and mid/high end road groupsets. Longstaff produce a customized freewheel mechanism for 2 wheel differential drive. They also make rear wheel trike conversions.