ZipDang
There are only two rooms on the main floor, separated by the large arch and, IIRC, pocket doors. The front room (behind the sofa), is what faces the street and one assumes is, or was the dining area.
In episodes where persons come in the front door, it can be heard by those in the kitchen and Jean even calls out to Lionel once that she is "in here", and he walks in with a shopping bag of sausages.
Beyond the kitchen door at the end of the hall is a stand of some sort, where the shopping basket is kept.
In one episode, Jean went out of the kitchen and was rooting about the airing cupboard under the stairs (this is also where she and Lionel hid for a bit during their wedding reception).
Speaking of the wedding reception, Jean is shown clearly trying to estimate how many persons could fit in the two rooms, something she wouldn't have to worry about if there was another dining room on the main floor.
It does seem that the kitchen table is rather large, and that most if not all meals are taken there, even when company is over.
Also in the episode where Penny, Jean and Lionel are having lunch in the back garden, one can see the kitchen window, and in some shots the steps that lead up to the kitchen.
It did strike one as odd, when finally seeing shots of the outside of the house, that the kitchen seemed to have been moved upstairs. However again, this is not all that uncommon with older London townhouses. Either an addition is added onto the back, or what was once a den or someother room at the back of the house is converted to a kitchen. Again, modern living without servants would be very tiresome with a kitchen in the basement. Especially if one has wee children at home.
Finally consider how much sunlight Jean's kitchen receives. That would hardly be possible if the area were located below stairs, and facing the back.
