Slow day so here we go.
First off for the shirts - if you want sustainable/quality I recommend Continental or Stanley/Stella. Both are GOTS and FairWear certified, both do organic cotton, Continental do bamboo as well.
For printing it all depends really. If you're selling then Streetshirt won't be any good, from what I recall their product range isn't great and price wise they're not practical. Not that they're bad, they're just retail focused, not trade.
For bulk orders (50+ of one design) you'll be looking at screen printing probably. £50ish setup costs but cheaper per piece and arguably better quality printing, depending on your point of view. Not good for lots of colours though, you pay more for each. I use Shirtworks for that stuff. Oxford based, they carry Continental and Stanley/Stella tees and decent customer service. Water based inks too, that's more or less standard though.
On bulk - not much point trying to provide your own shirts really. Some printers will take them on bulk orders but you've got the hassle of delivering them. Plus any bad prints come out of your pocket. If you let the printer supply shirts they cover duds and you're only paying a little more per piece.
For small runs go Direct to Garment (DTG) for printing. Cheaper option and I like the quality. Inkthreadable and Twofifteen are 2 UK companies that are decent. I use the latter. They drop ship too so you don't have to deal with stock. Business facing but fairly low key. Prices decent, Inkthreadable are cheaper but slower delivery times too I'm told.
If you've a bulk order in mind let me know, might be able to add it together with one of mine for a bigger discount.
