Under medium brake pressure, I can see the front drivers side hose moving about a little. It's a fixed caliper and I can see that the caliper isn't moving.

Is this because I've bought some crappy brake hose and decent hose would not do this, or is all braided hose typically of sufficient quality? Can't see any of the other hoses so can't comment on what the rest are doing. Any other reasons for it to do this?

They all do it, the inner hose deforms elastically until in transfers some of the load to the stiffer outer braid.

Rubber-textile hose also squirm under pressure but it is less visible.

Ah, didn't realise that. Does that mean it's a mistake to do the whole car with braided hose instead of just the last short connection to the calipers, or does it not really make very much difference?

I am the wrong person to ask I much prefer rubber-textile hoses for practical and quality and reliability reasons. The only faulty brand new flexible brake hoses I have ever encountered were big name braided hoses.

The main single seater use braided hose instead of solid pipe work is a whole brake circuit from master cylinders to callipers can be quickly exchanged as a unit with only a minimum amount of work and bleeding.