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I usually commute with flats wearing my working shoes, but I could at a push grab another bike that has SPDs, (in case there's a need to) so apart from taking a pair of shoes with me, and keep them at work (which may be an idea worth exploring) are there actually any clipless shoes that are subtle enough that I could get away with if I needed to?
My work isn't bothered about what I wear on my feet if that broadens the search a bit.
My work isn’t bothered about what I wear on my feet if that broadens the search a bit.

I'd just keep some shoes at work.
But make sure you don't upset any colleagues, who might then be tempted to take the traditional form of vengeance round these parts.
I have a pair of 5 Ten district spd's which don;t look too special....not sure f they make them any more though.
Adidas do the Velo samba. https://www.adidas.co.uk/the-velosamba-cycling-shoes/FY4486.html
But as above I just leave a pair of shoes at work.
You say there's no dress code. There's always a dress code.

or

or

Does your work have hard floors or carpets? Even the best flat pedal style clipless shoe is going to go 'click click click click' on hard floors.
Honestly, just buy a cheap, mega comfy pair of trainers for £30/40 max and leave them at work.
Or, if they're really not bothered what you wear...

https://www.biketart.com/clothing-c7/shoes-c51/giro-rumble-vr-mtb-cycling-shoes-p42758
These have been excellent for me - regularly cycle to work and wear in the office. Have lasted 4 years so far. Will be buying another pair when they eventually die.
Another vote for Giro Rumble VR. Very happy with mine.
Or, if they’re really not bothered what you wear…

Fixed that for you...
Not so spendy option
https://startfitness.co.uk/products/scott-mtb-ar-lace-clip-mens-cycling-shoes-black?variant=39826923618512
I just keep a pair of shoes at work to change into so no problems with wet feet from rain on way in or scratched floors from the cleats.
specialized 2f0 clips i own i`ve worn all day when lazy and on 'riding into work - going out for a lunch ride - riding home' days.
so apart from taking a pair of shoes with me, and keep them at work
This.
I used to keep a pair of shoes at work but covid meant a reorganisation of the office and they got rid of the lockers.
So I did what lots of people have suggested and bought some Velosambas (which are a little larger than my regular sambas I find).
