On the road you generally only need a 4mm and 5mm Allen key. For spokes , a Spokey does the job. Separate chain splitter; never found the ones on a multi tool work that well. Plus breaking a chain is a rare event.
Given the OP's parameters (didn't want separate tools) but also that they've found their £20 solution, I shall continue 'off topic' and stick to 'tools' not spares.
4mm and 5mm allen key.
Small flat head screwdriver. Chain tool (like
this one). Tiny light spanner to adjust spoke nipples (only ever used on others' wheels; mine self-built). 8mm spanner (v light) in case mudguards need fettling. Robust safety pin (for tyre cleansing and end of gear cable poking).
Anyone carrying a multi-tool is invariably carrying around stuff they have no need/use for, at least for their own bike, but I appreciate some like tools attached to one another (as opposed to in a v small plastic bag, renewed seasonally). Think this makes a bigger difference to those who have a stable of bikes and don't wish to kit out each one.
Rim brake axle nut and clamp bolt
Disc brake bolts (some need torx, mine don't)
RD and FD clamp bolts
RD and FD limit screws
Chain ring bolts
Seat clamp bolts
Seat post clamp bolt
RD hanger screws
RD bolt
RD pulley wheel screws
Bottle cage bolts
Cleat bolts
Mudguards securing bolts (at drop-out)
Mudguard stay bolts
Chain (failure)
STI and tyre tread poking