No election to worry about in the UK, so I suspect the decision-making is about trade-offs of the sort we've talked about here. The idea that "the economy is more important than grandma" is a loser, but so is the notion that all risk has to be removed before we try anything. We're already doing trade-offs in the food sector and with medical personnel.
First point. I think the UK is a little bit ahead of much of the US along the curve. Closer to New York (clear decline) than Texas (plateau). Two weeks behind Spain and maybe three behind Italy.
Second point. If you are going to do something, do it in low risk groups. The mortality risk for people under age 20 is very, very low, so it's more about the risk to older family members and teachers that is the concern.
Third point. We are about to get some useful data. Andalucia apparently opened its schools on May 18th.
Fourth point. School is a controlled environment. Insist on the use of face-masks and hand-washing to limit the risk of contagion, frequent tests to pick up carriers and people who have had it etc.
Put all of that together and I think Boris is early. I'd be looking at ginger steps when we have less than 1,000 cases per day.