There are many posts about US and Canadian Costco - they are fairly representative of UK Costco, but there seem to be some differences.
In the UK you can only join Costco if you work in a selected list of sectors including finance and education. (or in a genuine business, like a restaurant owner).
An annual fee of £25 + vat gets you an 'individual' membership for you and your spouse. I am single, but a nominated friend is my 'spouse' for Costco purposes, so we get membership for £12.50 each. A member may take in 2 guests.
Products seem to include many of the American mentioned stuff - but the fresh food does seem to be much more 'European', or even truly 'local'. At my local Costco (Bristol) I can get Cheddar aged in the Wooky Hole caves, for example.
My fave purchases are the loo roll (3 ply Costco own brand), the side of hot-smoked salmon at about £11, the chargrilled artichokes in oil (the chilled ones, not the ones in a glass jar), the guacamole (4 big sachets of excellent guac, with ingredients you'd use at home), the trays of canned chickpeas/sardines/tomatoes etc, and the big bags of pine-nuts. I reguarly buy a 'treat' to put on my desk at work, and this always begets orders for my next visit. (Mini cinnamon buns, jelly bellies, Poppycock popcorn etc.).
Those prices I've checked have been about 30% cheaper than Asda, though you do need to buy more than usual (eg 4 packets of haloumi, 12 cans of sardines). Wine comes in half cases or individually (for the pricier stuff). Fresh fruit and veg can be riper than you want it to be, considering how much you are buying.
The clothing section is great - lots of outdoor gear. The tools and kitchen equipment are fab - and the returns policy unquestioning.
I live alone, and visit Costco maybe once a month - is it worth it? I'm not sure financially - in that buying in bulk can lead to eating in bulk, and you do need to keep a sharp eye on supermarket offers to be sure you really are getting a bargain. But overall, I'd say it is definitely worth joining.