As of August 14, UK visitors are NOT obligatory to quarantine for two weeks upon arrival from Croatia. The epidemiological situation in Croatia is still good enough for UK standards. But the truth is, no one can predict if this will change in a week or two.
Croatia has recorded 208 cases of COVID-19 in the last 24 hours, which is the highest number of infected since the start of the pandemic. If the same trend continues for a week, Croatia will not satisfy the proposed UK standard of 20 infected per 100 000, meaning it will be a candidate for additional restrictions, like Spain and France.