If we lockdown for 12 months, then due to R0 being under 1, we will have next to no cases in the whole country apart from those that are imported. We would be in a situation like New Zealand, where thorough testing and quarantining of new arrivals would mean we would not need a lockdown at all. This would hopefully happen long before 12 months was up.
That's what will change - we will get cases low enough so testing is a replacement for lockdown. We won't have to wait for a vaccine.
Now, will our government cock it up is another question entirely, but the options are not "lockdown until a vaccine" vs "give up".