If you're not pushing the draw weights over about 120lb, linen is fine. Hemp is the best for very heavy bows because linen just ends up too thick. The £500 starting cost of getting a hemp license isn't worth it, on top of the many years you'll need to practice exactly how to ret and process the stuff you grow
Silk works well, but can be a faff to process. If you buy it as unprocessed cocoons be prepared for a very unpleasant smell. I'd stick with linen, especially if you can find actual, genuine Barbour linen. It stopped being made many years ago but there's a lot of the original Irish factory stuff out there and it's usually better than the European equivalent. That said, if it was stored badly by whoever's selling it you'll end up with rubbish!
18/3 is the way to go, individual lengths should break at around 15-25lb depending on quality.
Alternatively, try and find some good hemp thread. It is out there, but you'll need to go through a lot to find it. When you hit the right stuff, you'll be amazed at how much thinner your strings will be compared to linen for the same strength.