You have various retail chains - largest being DSG (Dixons Stores Group) which owns Dixons, Currys and PC World - they tend to do branded media, and their own "PC Line" brand a bit cheaper in tubs.
We have Staples and Costco, but they aren't a patch on US versions or them.
There are other catalog stores (eg Argos - with the GUS group owning many of them) - no wonder it's "ripoff Britain".
The cheap foreign stores sometimes have CD-R media (eg Aldi and Lidl )
There are a few online staores that have a good selection of media.
And then there's the Sunday computer fairs - a collection of small traders (often working from a garden shed) booking stands in a hired hall - several show circuits cover different areas, and if you stick with regular traders at a circuit you visit often, then you're normally ok.
I came unstuck once ... went to one I hadn't been to before, bought some RAM and it was the wrong type (said it was EDO, and it was only FPM) - first time the trader had been at that show, and last time too!
Trust the regulars - ONLY
I did get a bargain another time - Adaptec SCSI card for 10 quid!
OEM version, several on the table - I picked the latest date, with a SMALL BIOS chip, and it turned out to be flashable to the latest retail BIOS.
There's also a regular CD-R man at my local show, with a fine selection of branded and unbranded media, cased, slimline, tubbed and shrinkwrap, 10's, 25's, 50's and 100's
Even singles - great when I wanted to check if 90 or 99 worked in my drive.