The question is: how many infections happened before the quarantine was established? In UK the quarantine was started way to late. That the UK has in common with Italy, Spain and France. But in all these countries, a massive infection rate has been drastically reduced by the quarantine.

In Sweden, there seem to have been only very few initial cases and the virus seems to be slow to spread there - it probably helps that Sweden is over 80% larger than the UK and has only 1/6 of the population. Also, at this time of the year it is much colder than the UK, less reason to be outside in big groups.

If you check the infection graphs, in the UK the quarantine finally seems to have an impact on the infection numbers, which stopped growing a while ago. Sweden still seems to have growing infection numbers.