@ratemyflatplease there are lots of helpful ideas and some not so helpful about the service charge, which you can't change, or knocking out walls, which would be a lengthy and very expensive process unlikely to bring you a return on the cost. The configuration may not be ideal, but you bought it, so someone else will too.
The lease has been extended, so that's good. The service charge doesn't seem bad at all to me given it's London, but, and it's a big but, that depends on how much work has been done on maintenance, what is scheduled to be done (or will need doing soon) and what there is in the reserve fund. How is the communal part of the property maintained? Is it clean and smart to give a good first impression when viewers arrive? Talking of first impressions, yes, do take stuff off the top of the kitchen cupboards. Never put ideas into people's minds that a room lacks storage.
I would seriously look at the marketing. My feeling is that Foxtons don't have the best reputation from a buyers' point of view. They need to push all the positives hard and market it as a 3-4 bedroom flat with very flexible space. Ideally if there is a healthy reserve fund of moneys, let prospective purchasers know that straight away.
Obviously if you're looking at reductions then you don't want to spend money on stuff you won't want where you move to, but if it were me, for the cost of a weekend's time, I would try moving the furniture around to address the imbalance of living space that so many posters have spoken about.
Dining room next to the kitchen stays a dining room but combines with either the study furniture or the chairs and small table and lamp from the existing study to replicate the reading and relaxing area.
Main bedroom moves into the room with wardrobes. Sofa, sideboard, coffee table and TV move into existing main bedroom, potentially also you could potentially set up your study in this room, depending on whether it looks better there or in the room below.
Argos have a folding camp bed for £52 at the moment, dress the study as an occasional bedroom and the other bedroom as a spare/guest room. You then have one spare double bed to dismantle and store, would the mattress go underneath one of the other double beds?
The marketing blurb then becomes 'we use it like this, but when we bought it, it was a three bedroom upstairs and a study, which could be a nursery or occasional bedroom. You could plot the space out before deciding by using some storage boxes.
If that doesn't look good to you, all you've lost is your time.