Branch policies vary, I have encountered one large Tesco in the West Riding where the local manager has imposed a max 10 items restriction on all purchases (which is annoying as they come in trays of 12) but the majority of branches don't seem to be bothered.
I usually shop fairly late at night - used to do it overnight in 24h shops but they have mostly reverted to closing at midnight and reopening at 6am since the Great Pestilence.
I'm always careful not to clear a shop out of stock, I'll usually not take more than half of what they have on the shelves, to be fair to other customers, and so as not to annoy the managers. I'll usually drive round three to six of the largest stores that are within easy range of the Hall, and load up 12 to 24 cans or cartons of whatever we're buying at each one.
Tried using Costco once when Felicity had a card for one, it was more expensive than the supermarkets and had a much poorer range of goods, the only advantage was you could get golden syrup in 10 litre plastic bottles - but at about a third more than buying the same quantity in tins from Tesco would have cost.
I know some folk simply order it for supermarket delivery, but I quite enjoy the late night road trips round all the local big stored to buy in person.
Early expired / discount area is unlikely to have more than one or two cartons if they have any at all.
The way we use custard generally:
Supermarket value brand tins: The runniest type, good for pouring inside clothes at the start of a scene as it'll stain through and then leak quite quickly. Cheapest, but we don't go mad buying it as it's mainly intended for the very poor.
Supermarket own-brand (not value brand) cartons: These provide the bulk of what we use overall. Somewhat thicker and gloopier than value brand, great for external pouring, sitting in, internal pouring where you want to pour quantity, and good for hand-smearing too.
Proper brand (Ambrosia): The most expensive, much thicker than value or own brand, very good for hand-smearing becauase it's so thick. Not so good for pouring as a fair bit will be left in the carton unless you scoop it out with hands or a spoon. Good for underwear filling if you specifically want it to take a while to stain / show / leak through the recipient's outfit.
Similar to DM1.Major supermarkets (in person) have the most competitive prices for tinned goods and most other foods here in the UK. I've always stocked up over time and will certainly be doing so now I'm resuming occasional shooting. I'm located too remotely from any other option for it to be worth the journey so all other large volume goods are purchased online.
Black treacle and golden syrup: I used to have a monthly order from Amazon food for the 7kg cans which used to be about £12 each (saving about £20-40 for the same volume in multiple tins circa 10 yrs ago). The days of these prices have long gone and I just look out for the cheapest now. The cheapest 7kg appears currently to be £20-£25 (which is an awful lot) but the average trade stockist are now selling them for £40-£50.
The product I miss for stocking up (and whipping up) for pies was the £1 double cream alternative Tesco (and only Tesco) used to sell which was in cartons and could be kept un refrigerated for 5-6months. The generic product behind this is only available from catering suppliers now and it's so expensive I've gone back to a combination of real cream and Elmlea (buttermilk).
There's a uk site I've recently discovered called cakedecoratingcompany.co.uk . They sell huge, bulk containers of powdered cake mix etc. For example; 12kg for £49, a lot cheaper than buying it in the shops.