As far as I understand, the UK's strategy is essentially steepening the curve into one huge but thin peak, in order to minimize the time countermeasures have to be in place - which is the exact opposite of what "flattening the curve" is about.
Also, even if they do protect the vulnerable population, that only means they can stand a ~10x larger transmission rate. I don't think anybody knows how large it gets without some kind of action, but yeah, that would in theory make it possible to have milder actions.
That said, if they are indeed protect their population, I don't see how their plan differs from the rest of the world, that is trying to protect their vulnerable population too.
The earlier you start with countermeasures, the longer you can try them and see how they affect the spread rate. If you wait to impose them at the last minute, you will have to choose erring on too expensive measures or on letting your people die.
A suicide burn is when you're trying to land on a body (say, the Moon) by letting your ship fall freely, thus accelerating with gravity, for as long as possible, and then at the last second, you fire up your engines at 100% output and decelerate until hitting the ground. When timed right, you'll end up touching the ground exactly as your vertical velocity = 0. It's the most fuel-efficient way of landing, but if you miscalculate that time just a little bit, you'll end up smeared all across the surface.
Suicide burn works in rocketry because you can calculate all the forces to extreme accuracy and precision. Nobody would even think of attempting that if the error bars on your altitude or velocity measurements were +/- 1000%, or if your engine output was unstable. Trying to pull the epidemiological equivalent - timing countermeasures to keep NHS at exactly 100% utilization - seems like an impossible task that's bound to end with a disaster.