Both are true but for that 5% for whom it is serious it’s a lot when you take it as a percentage of 70,000,000 people. Roughly 350,000 in serious life threatening condition. Our intensive care can not cope with that number at once. Hence delay as in flatten the peak and stretch it out. Otherwise hospitals are overwhelmed and can’t even assess people let alone put them on oxygen etc. They would have to decide who gets help (who has the best chance of survival given resources) and who is left to die (or recover by themselves). It will be like war zone triage.
Sorry, 5% of 70,000,000 is not 350,000. 5% is 3,500,000.
Irrespective whether 40%, 50% or 80% of these guinea pigs for this "herd immunity" experiment catching the virus, or whether the "peak" is spread over 4 weeks or 4 months, nearly all these "5%" serious cases will die, because hardly any of them will ever get to be anywhere near an ICU. The following explains why.
If only 50% of the 3,500,000 so 1,750,000 need to stay in an ICU for say 10 days each, evening out equally over 4 months they would need 146,000 ICU beds. There are however only 3,700* ICU beds in UK, normally 80% occupied, i.e. under 700 free at any time. Even if everybody else was kicked out, 3,700 is c2.5% of 146,000. So 2.5% is their max chance of ever getting to meet an ICU bed.
That is without considering assisted ventilation. Experience in China and Italy indicates that since pneumonia is the main killer, mechanical ventilation equipment, preferably ECMO machines to allow damaged lungs to recover, are important lifesavers. We have
15 beds with such, good for 28 patients*.
Regarding herd immunity, you can read about it
here, none of the experts considers it worthwhile for COVID-19, because huge number will have to die before we get that as demonstrated above.
Perhaps the Government is waiting for the seriously ill to more or less fill the 700 beds. To hit that they will have to implement social distancing in about a week, because the number will continue to climb exponentially for a period after. The difference between that and implement now? A couple of hundred more will die. Presumably someone somewhere decided that price is worth paying to delay for a week - I can't see why, given late in also means late out.