Isn't it one of those things where "it depends", and there's no one simple answer? And the best answer will depend on whether you only care about price, or whether other factors are at play. For instance, someone only brewing stouts and milds will be less worried about the quality of hops than someone brewing NEIPAs - I've not heard much good about Worcester on that front, but YMMV.
For someone who only brews the odd beer using speciality malts then one of the places that does malts to measure may be cheaper, whereas if you brew them regularly then you may be better off buying a standard-size bag elsewhere. Recently I had cause to make a small batch of porter, and the best route was Brewstore in Edinburgh - their flat-rate pricing is good for more expensive speciality stuff and they sell in units of 10g which was perfect (and without the small-custom-order fees of say TMM) - and they're more generous than some on the weight they'll send by post not courier. As a shop they have higher overheads than others, but for this particular order they worked best (they also have other "oddities" like not charging extra to courier to the Highlands - not a big deal for most of us, potentially a big deal for a small minority, and they do some hops in 200g bags which helps the unit price).
And for some things you don't have much choice on "cheapest" as it's more a case of "only" - in fact my orders tend to be dictated by who has particular rare yeasts or obscure hops, and then making up an order to get free postage. Also if you really want particular speciality grains you often have to go to the people who get all their stuff from a particular merchant - BrewUK is best for Warminster, Brewstore has the full set of Simpsons including things like DRC which you don't often see elsewhere.
But frankly your best bet for sacks of grain is picking them up directly from local breweries - obviously it may require developing a bit of a relationship with them first although some are quite geared up as mini-wholesalers as the volume helps reduce the cost of their own inputs.
Staffs are an example of the latter, and I've been quoted £20 all-in for single sacks if I pick them up from a brewery closer to home.
If you ever find yourself near Peterborough then
Brewers Select could be an option. They seem to be winding down their hop stocks as the main hop merchants are getting more organised at direct sales -
Farams do 1kg packs but the list seems to be down at the moment, I guess because about now is when the new vintage hits the non-contract market.
Simply Hops (Barth-Haas UK) sell 5kg boxes, but again are a bit thin at the moment. And some varieties are only available direct from the farm -
Stocks Farm usually have at least one new experimental variety whereas
Bushel of Hops specialise in heritage varieties.
