Slovenia is one of the three EU countries that is forbidden by their own constitutions to extradite their own citizens to non-EU countries, which the UK became last Friday, at 23:00 UK time, midnight on the Continent.
Although the European Arrest Warrant continues to apply during the Brexit Implementation Period that lasts until 31 December 2020, the three countries have notified the European Commission that complying with the treaty would be unconstitutional for them. This however only applies to each country's own citizens. UK citizens, or other EU nationals, could still be extradited under the European Arrest Warrant during the transition period.
This means that if a Slovenian national committed a crime in the UK before fleeing back to Slovenia, they would escape criminal prosecution in the UK. Slovenian law, however, allows for Slovenian citizens who have committed a crime abroad to be prosecuted in Slovenia.