Data is from 2020-Mar-01 to 2021-Nov-18
Under 20 - No pre-existing condition: Total deaths in hospital with Covid-19: 17
Under 20 - Pre-existing condition: Total deaths in hospital with Covid-19: 59
20 to 40 - No pre-existing condition: Total deaths in hospital with Covid-19: 135
20 to 40 - Pre-existing condition: Total deaths in hospital with Covid-19: 675
So of total UK Covid deaths of 144,000 the potential total of deaths among under 40's of 886 is 0.62% - bear in mind this figure is people who died but had tested positive within 28 days. It's quite possible they were in hospital for some other issue and developed Covid-19 while being treated.
1 in 2000 people will experience severe adverse reactions, and for some of these reactions, age is not a factor. 1 in 18000 will die. Of the deaths most are elderly. But not all. Data are based on the Pfizer vaccine only since it has fewer adverse reactions.
Based on the above, a healthy person under 40 has a difficult risk assessment to undertake. The vaccines are relatively safe, but then there's not that much risk from Covid if you're a healthy 20 year old. Especially if you keep your immune system in good order.
Good data about severe adverse events is simply not openly published in the UK or other liberal Western democracies. The US VAERS system is open, but might be subject to abuse so I've used the Norwegian data. Norway has mandatory event reporting and mandatory follow up of severe events.
Data is from the NHS weekly Covid-19 statistics. I'm making the assumption that if a young person has severe Covid-19 they'll be in hospital. It's possible a few may die outside of hospital but the number is likely insignificant.