Not in my experience with pre-reg, warranty clock starts ticking from the day the bike is registered.
I would look at the price of the 1090R as a guide, when the 1190 got dropped it was replaced by the 1090 / 1290 models, one being higher spec and the other lower.
The 1290R is about £15k so the pre-reg probably looks like a real bargain against one of those, but the 1290 had a few upgrades over and above the slightly bigger engine (More things to go wrong) with the Keyless Ignition, TFT Screen and Cruise Control. So much more than a face-lift and new colours.
The 1090R sits a bit below the 1190R in that it does not have lean sensitive ABS and is down on power (125 vs 150) but in the real world it has a decent amount of power, I often swap between my 1090S and 1190S and really do not find the 1090 weedy, it does lack the mid range thump of the 1190, but it pulls well below 5k then has a small flat spot before pulling into a decent top end.
From what I have read from other owners (advrider) the 1090R has much better suspension than the 1190R, especially for off-road use, if that is your intended use I would really look at a 1090R deal rather than buy the 1190 because it looks like a bargain. Having said that a few folk around here have owned the 1190R and are probably far better placed to tell you about what they are like.
The fact the pre-reg is "new" when you buy it will do nothing to prop up its value after you have owned it a while, it will be worth no more than any other bikes registered at the same time with similar mileage.
I would look at the going rate for low mileage immaculate 1190's registered at the same time and then maybe add £750 - £1,000 to take into account it has never been used and everything on it is factory fresh.....
...but if that puts it close to 1090R money then I would try and do a deal on a 1090R, unless the extra power is really a big factor for you.
Maybe get a test ride on a 1090 first, this is very much the dna of the 1190, just lacking a chunk of power, but riding position and handling will be very similar.