Well there's a lot of useful things that 100 pounds can get you. Depending on what you consider incredibly useful, and what you do everyday. A quick list of the top of my head:
- good quality first aid kit
- old/new but shit smartphone
- pair of decent working boots
- shit ton of used clothes (atleast where I live, idk about the UK), to wear or as fabric
- set of screwdrivers and bits
- cheap drill, or good used drill
- used laptop
- used oscilloscope
- used all kinds of electronics really; TVs, displays, audio stuff
- thermal water bottle
- Self-defense stuff