"maybe because Sweden is a democracy and has no "authorities" that can guarantee the outcome of hypothetical future legal proceedings."
An extradition is not a legal proceeding, it's a diplomatic one and being a democracy they can very well agree not to engage in a diplomatic exchange beforehand.
Extradition is a legal proceeding between two countries with an existing extradition treaty. It only becomes a diplomatic proceeding if extradition is sought for charges not covered by the treaty.
Well, I don't know what treaties are to the United Kingdom and Sweden, so I can't speak to the relevant matter, but there is a general sense (insofar as "international law" can be thought of as an actual concrete thing) that treaties are laws. To the United States, for example, treaties are explicitly treated as laws (see http://law.justia.com/constitution/us/article-2/18-treaties-...). Most other countries take their treaty obligations very seriously, although the actual mechanics depend on each country's legal/political system.
In Sweden, it looks like these treaties at least are very similar to laws: extradiction would be handled by their judicial system and the decision would be ultimately up to their supreme court.
An extradition is not a legal proceeding, it's a diplomatic one and being a democracy they can very well agree not to engage in a diplomatic exchange beforehand.