Half or most items are the same price in plumbers merchants if you looked in detail. The main advantage for tradesmen is they get items on credit so pay the following month. We use screwfix alot as there prices are very competetive. If you know exaclty what item you want then online would be the cheapest option but if anything was to go wrong with the item you will need to send it back so this is not always practical.
PTS, Woodhouse & Sturnham, Plumb centre are the most expensive.