Although I usually lack adequate space for bulk purchasing, my mother-in-law (and her large chest freezer) can rarely resist the deals available from Costco.
For instance, a 5lb block of mozzarella cheese is about the same price as 1lb shredded from Tesco... now she has to put in the effort of grating it herself, but she has two days off a week anyway so she might as well take the time and save the money.
Costco's housebrand is called "Kirkland" and is actually very high quality for affordable prices. I can honestly say I have been buying Kirkland products for years and would highly recommend pretty much anything Kirkland brand.
We usually try to avoid this store on the weekends because the traffic from Lakeside can make for some pretty scary encounters however the parking lot is very large so at least finding a spot will be painless.
Membership is £50 a year but allows 2 adults so if you're savvy, you can possibly share a membership between two families like my MIL (she shares with one of her sisters).
If you are having a party, I highly recommend the cakes from the bakery here. We ordered the chocolate bundt cake and a carrot cake for a party we had and both were cheap, moist and sinfully delicious - especially the carrot cake!
Definitely worth the membership if you have a big family or a big event to host but make sure you check the hours first because on some days they open early to business owners and not to the public until noon or later. read more