Collected my prescription today and was told they couldn't source Eltroxin or Mercury Pharma levothyroxine (the same stuff, bar the packaging) in the 100 mcg dose. The 25 mcg tablets were available. With luck they'll be able to supply tablets that aren't Teva or Wockhardt. I'm in the East Midlands.
Update: Problem averted with thanks to helvella for the tip.