I'me with Terry on both of those HBCompany and CML for hops and yeast especially smaller amounts as postage is includedI mostly use The Homebrew Company.
https://www.thehomebrewcompany.co.uk
They are based in ROI and delivery takes a day longer than shipping from within the UK, but they do give good prices for stuff, usually the cheapest as far as I can see, and they have a big range on offer. There is a Forum discount of 5%, but you have to spend £70 to get free P&P.
And for hops and some yeasts I use CML like many others on here it seems, and they offer a Forum discount of 10%
https://www.crossmyloofbrew.co.uk
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