Most people become symptomatic at 5-6 days post exposure. But it ranges 2 -14 days. So quarantine is set at 14 days to cover the full range.
But it's not an even distribution, so most people who are going to fall ill will have done so by 12 days, so it's a judgement call about your numbers whether you risk cases getting through. If you have a high number of arrivals, then it's prudent to cover the whole incubation period as the even those the proportion is low the actual number is high enough to cause concern.
If you are going to need hospitalisation, then it's typically around 5-7 days after becoming symptomatic.
So being trapped and ill overseas is a scenario you need to be sure you can deal with - you will not be allowed to fly out if you are symptomatic (and it would be foully anntisocial to try to pretend you're not if you know otherwise). You would have to stay put until recovered, and your traveling companions may well have to go into isolation before being allowed to leave.
EHIC will only cover hospital expenses, on same footing as a national of that country (so often not free, but will be heavily subsidised). Up to December.
Cost of insurance which provides covid cover will increase enormously, because of the likelihood of need to pay out. Insurers don't care about preserving the travel industry.
Ironically, those with pre-existing health conditions may well be those least affected, as their premiums are already much higher because of increased possibility of requiring treatment when away.