What's something incredibly useful you can buy for roughly £100 or less?

Used cast iron pans - last lifetimes and can be used for so much. French press - coffee, tea, chicory, herbs, mushrooms ...

Honestly, I can't think of much stuff at a higher piece point that isn't specialized. Useful things tend to be relatively lower cost.

I carried a leatherman for years, but rarely used it. I find an adjustable wrench, picquick, pliers and box cutter more efficient and easier to use.

Although, I really want cool stuff.