Fill it with a piece of plinth or wood/mdf painted to match. I have slightly larger gaps at the end of a run in our kitchen - too small for regular cupboards and I didn't want a bottle rack, so my husband made a plain door with a push opening, it is perfect for trays & baking sheets. I can't think of items you may have in the utility that would fit the gap.

Whilst I prefer the sideboard in front of the window (to avoid the long line of furniture) it would block all the heat and I’m not sure the window is high enough for a console?? I would be tempted to add more of the ochre for curtains and if it’s far enough from the radiator an oversized plant in a woven basket. I’d pull the rug an inch towards the tv and move the far table (in this pic) back and add a table lamp. Perhaps also paint the radiator/s in the wall colour? You could add a textured throw over the arm of the sofa under the window? Perhaps in time replace the footstool with a slim brass/bronze and glass coffee table and add a knitted or fur pouffe at the ”bare” end?

I agree with others about artwork and/or books. Slim bookshelves for small paperbacks or you could use picture ledges for either nice cookery books or other books to display front side forward or for actual pictures! Could also add interest with high level shelf and plants that cascade down. Also look at picture lights that can be mounted on walks above pics/shelves you can get battery powered ones if you don't want to hard wire into walls. Also a rug/runner will help cosy things up.

Watering depends on the plant. A snake plant copes with dry conditions so I would suggest put your finger in the soil, if it is bone dry then water. If there is even the teeniest bit of moisture then leave it a bit longer. Peace Plants tell you when they need watering by drooping it’s leaves. I find that’s a good indicator to water all of your plants. I would say I water mine every 3 weeks or so and they are all doing well. Over watering is one of the most common reasons plants die. Here are some of mine.