Car dealers are pretty cute when it comes to assessing peoples moods.
You may have unwittingly given the impression that you are eager to get your hands on a IS250 as early as possible, even if it's not the spec you ordered.
The dealer maybe has noted this, and is going down the lines of, 'if we stand firm then we know he will cave in and pay the extra because he's eager'. This extra they are getting will help offset the earlier discount agreed.
I agree with the others and I would tell them that the current deal is right at the top of your budget and you cannot afford or justify the extra £1200.
The best way to tackle this is with a calm visit to the showroom, ask if there is any change in deal, and then just simply say, OK, see you in October and walk. I don't think I would threaten cancelling if it's a car you are really keen on, biting off nose etc.