Well if there's a 20% chance of a £1000 payout that's akin to paying everyone £200. Therefore premiums must be at least £200 + costs + profit.
In answer to your second question, theoretically a company should be able to insure one person, collect just one premium, and still make a profit. The more people they insure though, the more chance that this profit will tend to the expected level of profit, and that they won't make a loss - this is the law of large numbers, i.e. toss a coin 10 times and you might get 80% heads; toss a coin a million times and the number of heads will tend to 50%.