It's really no different than doing a fully mirrored vanity area in a bath. All you need is the glass shop for creating the template and installing it. Upsides are reflection of all good things,.......and the reverse: )

constant cleaning. like every little splash and speck of dust. If it's not 'flat' it will be noticeable. If it somehow gets damaged the entire thing needs replacing. What and where is the light source it would be reflecting? It's not a look I'd go for. I'd find other ways of solving the light issue. If you do decide to go for it, get a designer in board and do it well

I'm thinking it won't be completely clear mirror more like this photo which is sort of grey. Not sure if there is a name for this..

I guess if you don’t mind seeing 2 of everyhing on your counter creating a cluttered look go for it with the things mentioned above . IMO more lighting would be a better choice, so easy to add with LED minimal LED track or something else .

Undercabinet lighting - not the cheap kind I bought - but installed by a pro preferably.

Sounds great…you have to clean any backsplash all the time, anyway, so why not mirrored?

The kitchen cabinets will be new so underside will be good. Also I'm the type of person who has nothing on their worktops apart from a kettle and a toaster and that will be tucked away in a corner. There will be LED strip lighting under the cabinets but there is column in the middle of the kitchen blocking some natural light, hence thought during daylight mirror would be just add another dimension of light....

PRO / Dallas Renovation Group: "This backsplash is actually fabric that is behind tempered glass, that is offset from the walls, with metal offsets. As such, you could change the fabric as desired, and it's very easy to clean.
Offsets are typically metal, and set the material (in this case glass) off from the back wall.
The fabric is on cardboard, with the silhouette of the glass (or other desired shape).
The nice thing about this setup, is you can change this fabric as you change your kitchen."

Stainless steel backsplash?

Charco Design: "we painted the back of a seamless glass backsplash!"

Divine Design + Build: "Four inch granite back splash / the bead board is painted with semi-gloss, which can be wiped."

back-painted glass panels

@everdebz thanks for all your suggestions. Particularly like the painted tempered glass look! I'm thinking you too are not feeling my mirrored look?

I haven't done any of these. Whatever style your cabinets are, these might or might not work - good 'luck'!

We posted at same time. It might look good, but it'd have to be so well done, and fitting with the house and style of kitchen, etc... shown in digital form ahead of time... seems I don't understand the concern yet -- if you'll have good lighting under the cabinets. I also dislike the shadowed work area of my counter.

Are you possibly choosing too much dark: dark flooring, cabinets, etc..??

@everdebz my cabinets will be an off white colour with an engineered stone counter not decided on colour yet but probably like a Carrara marble. Floor is large long tiles that look like parquet in a taupe/grey color.

Not a glam/orous type... handles, etc... fancy chandelier?

@everdebz no handles see pick below. Large LED spot lights in the ceiling with some sort of hanging light fixture over the island.

We did this one time for a client. We used cabinet end panels to create a bottom for the cabinets. We recessed ‘puck’ lights into the panels, the panels were attached through the floor of the cabinet.

"Your mirror backslash will reflect the underside of your cabinets
Nope. That's not how reflection works. The angle of incidence is equal to the angle of reflection. In other words from standing in front of the counter looking down at the mirror, you will see what's on the opposite angle (flipped horizontally) to your gaze, hence you'll see what's on the counter

The underside of the counter will not be reflected unless you bend and look from a point you can see it directly from in any case

Ahhh, so you CAN teach an old dog new tricks! :-)

Haha ;)