There are models. They absolutely have a great idea that we'll be getting back to normal in late spring - at least for the summer. They want to do it earlier because Conservatism, but it's unlikely.
They could, and IMO, should say, if we can meet condition x, this is when y unlocks. That would focus people's minds, in my opinion.
This is what happened in the first lockdown. Then it got thrown out. TBH, the temptation to massage numbers makes it almost meaningless.
And honestly, Boris doesn't like the idea of setting out the future because he wants to react to the landscape and make decisions as late as possible.
He's not the only one, I reckon. Many leaders making this same choice. I think they are outrageous bastards, letting people get their hopes up and then see them dashed, letting the crazy sorts of stories that sell papers set the narrative, letting people believe what they want, and seeing everyone disagree on the basics, and believe something different about the reality now and the future prospects.
It's not that hard. There is a limited range of possibilities that are likely to transpire. We can say what is likely in each case, and make the point that the unexpected can always happen, and we would have to think again if that was the case.