I would place a bet that they will produce some "reasons" for deadline extensions way up until no one can afford petrol anways anymore...
See Germans exiting nuclear and coal based power generation on lessons on how "finding reasons" works. We could be rid of both by now, but we are not - because of "reasons".
We'll see, I guess. My local Member of Parliament (Labour) is a former executive with (I think) Peugeot, and has a keen interest in the car industry, transport and green issues.
I receive updates from his office on his parliamentary work and, as far as I can see from following the business of these committees, preparations are already will in hand.
Whether the government will meet the revised, earlier, target is another question, but I would be very surprised if they "found reasons" to cancel the plans altogether, particularly since all the major British auto manufacturers, at least, are planning their production on these assumptions, and they (and the government) have already spent a lot of money preparing for the change.
It's one thing to decide to stick with nuclear or coal because the costs of decommissioning the old power plants and building new ones are too great, or building new power plants is taking longer than expected -- that's sticking with something you've got -- but telling manufacturers that they won't be allowed to sell new petrol or diesel powered cars after a particular date is something rather different, since even if the government change their minds later on, the manufacturers aren't likely to revert to old models when they've spent a lot of time and money on phasing them out of the production process and introducing electric and hydrogen powered ones, I would have thought.
Even if they find they can still sell cars powered by fossil fuels, at least for a while longer, they'll no longer be making them.