There’s nothing on a par with a BMW X1 left on the scheme anymore so, yes, you will probably miss it, but if you are finding the petrol cost to be too expensive for you then I would think that you have no choice but to change?
We have the same car as you but the newer facelift model with four wheel drive. We are averaging 40mpg now in the winter (cold starts, heater cranked up, heated seats on etc), but were getting 44mpg during the summer, which IMHO is excellent for a powerful petrol 4WD auto SUV and is in fact better than we were getting on our previous VW Tiguan diesel.
Obviously everybody has to live within their means, so a self charging hybrid might be cheaper for you to run. However, keep in mind that the Advance Payment (AP) price that you be paying for your X1 is an absolute bargain compared to the AP’s for cars now. If you cancel and order something else, you could easily end up paying a bigger AP for a car that’s nowhere near as good as what you have on order and obviously any extra AP that you end up paying will eat into any fuel savings that you make. A difficult decision for you.