They certainly can charge in absentia and issue extradition warrants afterwards - read the UK high court judgment. The problem is that as soon as they charge th

They certainly can charge in absentia and issue extradition warrants afterwards - read the UK high court judgment. The problem is that as soon as they charge they have to share evidence to get an extradition warrant, and Assange could have beat the request on substantive grounds by demonstrating that there was no crime under British law, something that would have been particularly easy since the Swedish prosecutor would be required to share evidence, including the exculpatory texts. They did not charge because the requirements to extradite for "questioning" are much weaker and require no proof, just a statement from the Swedish prosecutor that cannot be challenged on any substantive grounds.