This is a really awkward space! What about trying to build a unit into the corner which seems to be dead space. This could house the television and other bits like dvr,amp etc. The unit could then extend out towards the door in length and in height either up to the start of the window or up to the start of the blinds. I would suggest drawers for toy storage and books above. In this way you leave the windows clear for light plus it looks like you have a pull down screen so the tv would be out of the way of that. This arrangement would also hide all those wires and cables! Ikea have some good examples of storage which you could adapt or copy, such as Besta which illustrates the extended toy storage. http://www.ikea.com/gb/en/catalog/products/S39927155/

You have lovely pieces to work with. To my mind, it appears you have a long and a short MCM credenza/buffet. I'd prefer them switched and then move the art pieces so they take up no more than 75% of the credenza's length. Also keep them closer to the furniture and lower than the door and window frames if possible. Red is a wonderfully appetizing color and I'd love to see it on the dining room wall as well, although I know it may make the air unit more obvious. The dining wall is the one I'd fill with a bookcase for the cookbooks. As a one-time owner of nearly 100 cookbooks, I found they made for great dinner party conversation, along with an unabridged two-volume set of the English language dictionary. An IKEA bookcase could work well and hold plenty of books. The white shelf system with the red wall behind and the backs of the shelves painted red would look great! Or you can paint the bookcase red and leave the wall white. Group books by color for another touch of fun! Don't forget to show off some great "foodie" accessories too.

Medicine cabinets have come along way in recent years. The best option is to get a frameless style and integrate it into a wall of mirror. You can mount sconces thru the mirror on either side of the cabinet. If you have walls on either side of the sink, you can put the cabinets to the side, integrated into mirror again. If you have an open wall in your bath, yo can get an extra tall medicine cabinet to hold loads of stuff. There are loads more on houzz photos. Just search medicine cabinet. Sometimes you have to hunt for the cabinet because they are so well integrated. Robern makes some of the best frameless styles, but there are a ton of others on faucetdirect.com. Many even come with integrated lighting for a very modern look.

I agree you probably need to centre the vanity and the cabinet on the wall or they could look a bit odd! You don't say why you don't want a space between the window and the vanity - just my opinion, but I would think 300mm could be quite useful for a bin, a clothes basket or box, or a small stool for putting clothes on? The only alternative to leaving a space, is to fill it with something as the response above suggested. To avoid it interfering with the window, you could make this something lower, so it sits level with the top of the vanity. This could be built in and the wall lining put over it to create a single "shelf" level with the underside of the vanity top, or if you can find some freestanding open shelving of the correct width, which could also be cut to the correct height, you could wedge that between the wall and the vanity side when fitting the vanity. Just bear in mind also, that you'll lose some of your towel hanging space if you put something too high in the space.