He'd already allowed for his time etc in his time element of his estimate and specifically mentioned time for buying materials in the narrative of his, very comprehensive, quote as to what was included in the time fee quoted. In fact, on the morning he arrived, he went for the materials on his way, and as a result of that, arrived about an hour later than he did on the other days, and he charged a full day's labour for that, so his time for collecting materials was part of his daily bill, and the receipts he provided were dated that morning too! So he hadn't made any special journeys on previous days!
His van costs, ladders and other "tools of the trade" should be incorporated into his hourly rate. If, as you say, we'd bought materials ourselves, he wouldn't have made any extra/hidden profit.
I hate the way that businesses make "hidden" profits - they should be open and upfront. If they want to charge a "handling" fee, fair enough, be upfront about it.
As it were, we acted that way with him because we couldn't believe that he'd use £200 of materials for the job, as we did a quick google to gauge an idea of the prices for the materials he said he'd be using and couldn't get anywhere near £200 for what was basically a bit of masking tape and some plastic sheets (for covering windows), along with a tin of exterior paint and a tin of exterior primer. There was no external wood to paint as it's all PVC (windows, soffits, fascias etc) so no undercoat/gloss needed. If he'd have quoted £100 for materials, we'd not have given it a second thought. But to make a £100 extra/hidden profit on buying £100 of materials is taking the piss!