(When I read your title I thought you meant a single plate cost £5, rather than being part of a £5 set.
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Some of my things which have stood the test of time are: a black metallic computing trolley from the 1980s, which has been taken apart and put back together lots of times for various house moves. Still as solid as ever, no pieces missing.
A 4-way trailing socket I demanded for my bedroom when I was 15. The neon light flickers, but the sockets work.
A Nike fleece scarf from a sports shop in 2001.
I kept my first computer from University (Windows 98) going for ten years. The non-flat screen monitor stopped working, but the PC was fine, until it became impossibly slow (what it really couldn't handle was when you type something in, and suggestions appear underneath). I still say Windows 98 was the best, everything since then has been difficult use.
I've had the same mobile number since 1998 - I thought I would probably be forced to change it at some point. It's an easy one to remember, so I clung to it!
Video player from 2001, used occasionally.
A cat, which wandered into my friend's house, and made herself at home. She wasn't chipped, so my friend (who had two cats already) pleaded with me to take her! Nearly twelve years on she's very much around, despite several house moves.