Electrical wholesaler, if you order in box qtys, but not if you want boxes splitting.
Copper tube: Wickes for a bundle of 10, except had to use B&Q for 28mm. (I would not be surprised if these aren't the cheapest anymore)
Copper fittings: bes.co.uk
Radiators: I didn't want cheapest, and used a local independant plumber's merchant as they had next day access to every possible size direct from manufacturer just up the road (including triple panel ones), so I could size each exactly as I wanted, not limited to stock sizes kept by the sheds and larger plumber's merchants.
TRV's: Same local independant plumber's merchant gave me best price. Nowadays, ebay seems to be cheapest for the TRV4.
Boiler: call around lots of plumber's merchant's. Easily found a factor of 3 difference in price for the (less common) model I was after.
Light fittings are a matter of taste. The sheds are probably the cheapest, but also the least inspiring.
Pipe from local plumbers. Screwfix and Toolstation are expensive for pipe. Their fittings are reasonable. Thermostats, Honeywell from ebay. the best make, and you can get them there for the typical shop price of other makes. Electrical supplies, your local independent electrical wholesaler, TLC are competitive, or, try one of the bigger brands, CEF, ERF etc, and ask them if they'll give you trade prices. B+Q are not cheap for anything IMO.
I doubt someone just buying enough kit for just one rewire will do as well at an electrical wholesalers as they would do at TLC for cable prices.
I do pay less than TLC prices for cable but I do have to have a minimum spend per month to keep the prices down (or buy 10x 100m rolls at a time).
Although there are exceptions. One bloke got an extra 20% his prices off by keeping to his promise of sending his wife in topless to collect and pay for the gear.
I find TLC are actually quite good with discounts... I don't order that much from them, but have placed a handful of orders for 500 quids worth of stuff at a time in the past. These days if I go in and buy anything, as soon as they get my postcode or a look at the credit card etc they normally knock something off. E.g. popped in to get another length of dado trunking (moral, don't let a client measure how much is needed - especially when they measure in inches, and then give you that number but tell you its in cm!), and they knocked off a few quid which probably amounted to 15% or more off the catalogue price.
Since Toolstation opened a branch locally about a year ago, I feel like I've died and gone to heaven :)
expensive and have only used them for relatively obscure stuff that Toolstation/Screwfix don't carry. Though I must admit it's their website which is the biggest turn-off for me.
Their attraction is the range of stuff they carry. Having said that prices were not bad last time I ordered a reasonable amount - but that was a few years back now.
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