You are becoming the Bunting Queen. Perhaps
@DementedFairy is the best one to assist with bunting.
(cough, cough)I think it depends on how permanent you want it to be and how long each continuous length will be-the structural needs of say 5 meters is different from 50 meters.
Will the bunting get cut down into 3-5 meter lengths to be given to guests or bridal party members as mementos after the party? Am I creating a new nightmare by even suggesting that? I think it would be a cool souvenir.
When I made my bunting to use every year at Hallowe'en I used cotton twill. Bias is pretty stretchy and if you're stringing up long lengths it can get pretty droopy. Several sources in the UK list bunting tape, which is variously colored cotton twill. Some comes with a crease woven down the middle, like fold over elastic has, to make folding it easy. Another less expensive option might be rayon seam tape. It comes in 100 yard rolls for about $10, or 7 UK pounds. It would handle as much weight over a big distance.
You've just made me think about the structural engineering needs of bunting.
Weight of flags over x distance, indoor or outdoor, where wind and rain might come into play, etc. It's too much to think about.