My initial thoughts just going from previous experience during our extension planning are:
What is your view from the window if you put it and the sink on the other wall? How would this affect the light coming into the room?
The two sets of bifolds will make the room brighter, but also less private at night if you have the lights on watching TV - are you going to be overlooked and feel self concious when sat in there at night?
If so, think about possible window coverings/blinds and how they would fit onto the walls around the bifolds.
Are you going to need to do anything outside to accomodate the bifolds - steps down, moving or laying of new patio etc, these will all add onto the cost, as will an extra set of bifold doors.
Do you really want bifold? We thought that was the way forward for our kitchen, but then decided on multiple overlapping sliding doors for a few reasons:
The multiple sliding doors were available with larger panels, so more glass and less frame - as they spend the majority of their time closed, its nice to see more of the garden and less framework.
Sliding doors can be opened just a fraction and still vent to outside without needing to open a whole door leaf, useful if you may have small pets you want to keep inside, but still allow fresh air in, or vent the room when its raining etc.
The sliding door systems allowed for a fully flush finish between the indoors and outdoors. (see attached image).
As for the hob on an island, we did the same but made the mistake of not having gas piped there in addition to the electricity. We are now forever stuck with electric hobs - at the moment we like the induction hob,but if we ever wanted to go back to gas, we are stuffed. Make sure if you do have the hob on the island, you have both services run in - and its probably worth running two electricity supplies in case you ever want an aditional wok hob or somethis else. For the few quid it will cost during the build, its always there if you need it.
In one of your drawings, your fridge is on the opposite side of the room to your sink, forcing you to walk round the island to get from one to the other. This will get tiresome quickly. Its most likely that your kettle will live near the sink, and if you want milk in your tea/coffee thats a 6m walk back and forth every time you put the kettle on. Make sure your cooking area, sink/dishwasher area and fridge/food storage are relatively close and create a "working" zone, so you arent constantly walking to eachend when prepping food.
It also stops people having to walk past the hob (which may be in use) to go from sink to fridge.
Where is the Oven? if in the island along with the hob, make sure you leave enough room between the open oven for the door to open and the person to bend over and put stuff in or take stuff out without hitting their backside on the units opposite.
Give more than enough room to circulate and allow two people to pass each other in the spaces between units. You dont want to be creating a 650 sqtf expensive extension and kitchen and then find you create a one way system in the kitchen becuase people travelling in opposite directions cant get past each other.
Think about lighting and if there is more than one entance/exit to the room you want to e able to switch these lights off inmore than one place. Do you want to be able to controll the lights from an area not near an exit, the seating area perhaps.
Spend a lot of time thinking about the design and future proofing in case you change the layout of the lounge furniture. ie. put a tv cable in the wall on the opposite side, even if you dont put the tv connection point on the wall, at least you know if you decide to move the furniture and tv around, all you have to do is punch a hole in the wall and hey presto aerial cable where you want it.
Im sure there were other items we thought of during our planning, and some that we missed but hopefully those will give some food for thought.
Also, try downloading google sketchup (i think its still free), you can then easily draw plans to scale and see how things look from different angles.