@Meredithgrey1, As I understand it, it's all about the reproduction number (R). So, if they can see after a period of time, that the R is well under 1 (which means you infect less than one person) and it stays there, under lockdown, they can slowly, very slowly, start to lift a few of the measures. This is what Spain are doing, yesterday, for the first time in 6 weeks kids under 14 were allowed out for an hour with an adult. They've been stuck indoors for 6 weeks. Next weekend, Spain are going to allow adults out for exercise. Then a week after that they will relax a further restriction.
All the time, the focus will be what is happening with the R number. If it rises, they will reintroduce measures again.
This is basically what Boris said - keep the lockdown, make sure the R number gets to where it should be and stays there, before moving into phase 2.
If we all went back to normal tomorrow, the R number which is currently going down, would shoot back up, and we'd have a second peak before we're out of the first. If enough people continue the flout social distancing, it will go up, and lockdown will be extended, and possibly more restrictive.
Whilst under lockdown, the R number will decrease to a point where transmission chances are very low.
Whitty said last week that some social distances will stay in place for up to a year, or when there is a vaccine. Those measures will mean life is different to how it was in February, but life will not be as restrictive as it is now. As long as people do what we need to do now, then this lockdown will end earlier but if people flout the advice, the R goes up, the lockdown extends.
That's how I understand it, anyway.