What upgrade will make the most difference?

So, hypothetically, imagine you had around £100 to spend to upgrade something on a road bike. What would you spend it on?
If you want a more specific scenario then imagine you have a Ribble roadbike that's maybe five years, with an aluminium frame and other bits, carbon fork, Mavic Cosmos wheels and a Sora 8-speed groupset. And relatively new tyres. And you are concerned more with comfort than with weight or speed.
I've thought about a carbon seatpost - but I don't know how much difference this would actually make? I'm imagining it would be more comfortable than alloy.
Or perhaps I'd be better off buying a new pair of shoes - I currently have a fairly cheap pair from Decathlon, but they do fit me really well (I have freakishly wide feet and find shoe buying quite tricky).
If you want a more specific scenario then imagine you have a Ribble roadbike that's maybe five years, with an aluminium frame and other bits, carbon fork, Mavic Cosmos wheels and a Sora 8-speed groupset. And relatively new tyres. And you are concerned more with comfort than with weight or speed.
I've thought about a carbon seatpost - but I don't know how much difference this would actually make? I'm imagining it would be more comfortable than alloy.
Or perhaps I'd be better off buying a new pair of shoes - I currently have a fairly cheap pair from Decathlon, but they do fit me really well (I have freakishly wide feet and find shoe buying quite tricky).
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05 Spesh Enduro Expert
05 Trek 1000 Custom build
Speedily Singular Thingy
Specialized Roubaix SL3 Expert 2012, Cannondale CAAD5,
Marin Mount Vision (1997), Edinburgh Country tourer, 3 cats!
invest the 100 quid