My builder rang the builder's merchant, put the order in, handed the phone to me and I paid them directly. I never gave the builder money for materials.
At the end of every week he invoiced me for his time which was clearly detailed as £125 per day x 5 = £625 then separately listed at the bottom for any help he had, so inexperienced labourer £50 per day, experienced labourer £80 per day. And yes I could tell the difference. I transferred money directly into his bank account whilst he was cleaning up, so he knew I had paid before he left my house.
I paid all the sub-contractors directly (electrician, plasterer, roofer, door and window chap etc) and the entire thing came to the same quote the builder gave me in the first place.
He turned up when he said, there were no delays in any of the building works, yes he is amazing, and yes he was always booked up. No he is no longer available, after 20+ years he is doing something else.
But is there a contract in place, insurance etc for your builder doing your works? I would be incredibly wary of handing over 50% of the money, it is incredibly risky.